If I choose two open sets $A$ and $B$ as depicted on Wikipedia here:



then I have an isomorphism between $H_n(A \cap B)$ and $H_n(A) \oplus H_n(B)$ because the two tubes in $A \cap B$ are disjoint.

OK, so far so good. Then I write down the Mayer-Vietoris sequence and try to compute $H_n(T^2)$ but it seems to me I don't have enough information to do so. Can you confirm this?

I then computed the reduced MVS instead and used that to compute $H_n(T^2)$. What I would like to know is if this is the only way or if there is a way to do it with MVS directly, without the reduced MVS. 

Many thanks for your help, I appreciate it.

Maybe I can add some details to Dylan's answer.

First of all, with the open cover you have chosen, you have

$$
A, B \quad \cong \quad \text{cylinder} \quad \simeq \quad S^1 
$$

and

$$
A \cap B \quad \cong \quad \text{disjoint union of two cylinders} \quad \simeq \quad S^1 \sqcup S^1  \ .
$$

Hence

$$
H_n(A) = H_n(B) = 
\begin{cases}
\mathbb{Z} & \text{if}\ n=0,1 \\
0          & \text{otherwise}
\end{cases}
$$

and

$$
H_n(A\cap B) = 
\begin{cases}
\mathbb{Z}\oplus \mathbb{Z} & \text{if}\ n=0,1 \\
0          & \text{otherwise}
\end{cases}
$$

Hence, in particular, for $n\geq 2$, $H_n(A) = H_n(B) = H_n(A\cap B) = 0$. Thus, from the MVS,

$$
\dots \longrightarrow H_n(A)\oplus H_n(B) \longrightarrow H_n(\mathbb{T}^2) \longrightarrow H_{n-1}(A \cap B) \longrightarrow \dots
$$

which, for $n > 2$, is just

$$
\dots \longrightarrow 0  \longrightarrow H_n(\mathbb{T}^2) \longrightarrow 0 \longrightarrow \dots
$$

follows that 

$$
H_n(\mathbb{T}^2) = 0, \quad \text{for} \quad n> 2 .
$$

So, again we can focus on what happens for $n=0,1,2$. For $n=0$ there is no big deal, because, since $\mathbb{T}^2$ is connected, 

$$
H_0 (\mathbb{T}^2) = \mathbb{Z} \ .
$$

For $n= 2$, we look at this piece of the MVS:

$$
\dots \longrightarrow H_2(A)\oplus H_2(B) \longrightarrow H_2(\mathbb{T}^2) \stackrel{\partial}{\longrightarrow} H_1(A\cap B) \stackrel{(i_*, j_*)}{\longrightarrow} H_1(A) \oplus H_1(B) \longrightarrow \dots \ ,
$$

of which we know all the groups, except the torus' one:

$$
\dots \longrightarrow 0 \longrightarrow H_2(\mathbb{T}^2) \stackrel{\partial}{\longrightarrow} \mathbb{Z}\oplus\mathbb{Z} \stackrel{(i_*, j_*)}{\longrightarrow} \mathbb{Z}\oplus\mathbb{Z} \longrightarrow \dots
$$

Now, as Steve D. pointed you out, despite the two last groups being isomorphic, it doesn't mean that the morphism $(i_*, j_*)$ between them in the MVS, induced by the inclusions $i: A\cap B \longrightarrow A$ and $j: A\cap B \longrightarrow B$, is an isomorphism. And actually it is not. (If it was, then $H_2(\mathbb{T}^2) $ would be zero.)

At this stage, you need to compute who this $(i_*, j_*)$ is. For this, you choose $1$-cicles generating the homologies of $A, B $ and $A\cap B$ as follows: for each cylinder of the intersection $A\cap B$, take an equatorial circumference. Name their homology classes $\alpha$ and $\beta$. So, actually, those $\mathbb{Z}$ in the piece of MVS depicted above are the free abelian groups generated by $\alpha$ and $\beta$:

$$
(i_*, j_*) : \mathbb{Z}\langle \alpha \rangle \oplus \mathbb{Z}\langle \beta \rangle \longrightarrow \mathbb{Z}\langle \alpha \rangle \oplus \mathbb{Z}\langle \beta \rangle
$$

And now we compute:

$$
(i_* , j_*) (\alpha , 0) = (i_* , j_*) (0, \beta )  = (\alpha , \beta)
$$

since $\alpha = \beta$ in $H_1(A)$ and $H_1(B)$.

Hence, in terms of these basis, our morphism $(i_* , j_*)$ can be represented by the matrix

$$
\begin{pmatrix}
1 & 1 \\
1 & 1
\end{pmatrix} :
\mathbb{Z} \oplus \mathbb{Z} \longrightarrow \mathbb{Z} \oplus \mathbb{Z}
$$

Hence,

$$
H_2(\mathbb{T}^2 ) = \mathrm{im}\ \partial = \mathrm{ker}\
\begin{pmatrix}
1 & 1 \\
1 & 1
\end{pmatrix}
= \mathbb{Z}\langle \alpha - \beta \rangle = \mathbb{Z}
$$

As for $H_1(\mathbb{T}^2)$, let's focus on the following piece of the MVS:

$$
\dots \longrightarrow H_1(A\cap B) \stackrel{(i_*, j_*)}{\longrightarrow} H_1(A) \oplus H_1(B) \stackrel{k_* - l_*}{\longrightarrow} H_1(\mathbb{T}^2) \stackrel{\partial}{\longrightarrow} H_0 (A\cap B) \stackrel{(i_* , j_*)}{\longrightarrow} H_0(A) \oplus H_0(B) \longrightarrow \dots \ ,
$$

where $k_* - l_*$ is the morphism induced by the inclusions $k: A \longrightarrow \mathbb{T}^2$ and $l : B \longrightarrow \mathbb{T}^2$.

Again, we know all the groups except $H_1(\mathbb{T}^2)$:

$$
\dots \longrightarrow \mathbb{Z}\oplus\mathbb{Z} \stackrel{(i_*, j_*)}{\longrightarrow} \mathbb{Z}\oplus\mathbb{Z} \stackrel{k_* - l_*}{\longrightarrow} H_1(\mathbb{T}^2) \stackrel{\partial}{\longrightarrow} \mathbb{Z}\oplus\mathbb{Z}  \stackrel{(i_* , j_*)}{\longrightarrow}\mathbb{Z}\oplus\mathbb{Z}   \longrightarrow \dots
$$

And again, we need some knowledge of the morphisms involved. I claim that, taking as generators for $H_0(A)$ and $ H_0(B)$ two points $p,q$, one in each component of $A\cap B$, you have also generators for $H_0(A\cap B)$ and, with these generators and similar computations that the ones we have already done, the second morphism $(i_*, j_*)$ can also be represented by the matrix

$$
\begin{pmatrix}
1 & 1 \\
1 & 1
\end{pmatrix} :
\mathbb{Z} \oplus \mathbb{Z} \longrightarrow \mathbb{Z} \oplus \mathbb{Z}
$$

Now, you get a short exact sequence from that piece of the MVS in the standard way:

$$ 0\longrightarrow \mathrm{ker}\ \partial \longrightarrow H_1(\mathrm{T}^2) \longrightarrow
\mathrm{im}\ \partial 
\longrightarrow 0
$$

But, on one hand, 

$$
 \mathrm{im}\ \partial = \mathrm{ker}\ (i_*, j_*) = \mathrm{\ker}\
\begin{pmatrix}
1 & 1 \\
1 & 1
\end{pmatrix} 
= \mathbb{Z}
$$

On the other hand,

$$
\mathrm{ker}\ \partial = \mathrm{im}\ (k_*-l_*) =(\mathbb{Z}\oplus \mathbb{Z}) / \mathrm{ker}(k_*-l_* ) = (\mathbb{Z}\oplus \mathbb{Z})/\mathrm{im}(i_*, j_*) = \mathbb{Z}
$$

Hence, we have the following short exact sequence:

$$
0 \longrightarrow \mathbb{Z} \longrightarrow H_1(\mathbb{T}^2) \longrightarrow \mathbb{Z} \longrightarrow 0 \ .
$$

Thus

$$
H_1(\mathbb{T}^2) = \mathbb{Z} \oplus \mathbb{Z} \ .
$$

EDIT. Some more details. Inclusions 

$$
A \stackrel{i}{\longleftarrow} A\cap B \stackrel{j}{\longrightarrow} B
$$

induce morphims

$$
H_1(A) \stackrel{i_*}{\longleftarrow} H_1(A\cap B ) \stackrel{j_*}{\longrightarrow} H_1(B) \ .
$$

Hence a morphism

$$
(i_*, j_*) : H_1(A\cap B) \longrightarrow H_1(A) \times H_1(B) = H_1(A) \oplus H_1(B) \ ,
$$

whose value on $1$-cycles $\sigma : \Delta^1 \longrightarrow A\cap B$ is

$$
(i_*, j_*) [\sigma] = (i_*[\sigma], j_* [\sigma]) = ([i\circ \sigma],[j\circ \sigma]) .
$$

Hence, if $a: \Delta^1 \longrightarrow A\cap B$ and $b: \Delta^1 \longrightarrow A\cap B$ are $1$-cycles representatives of the homology classes $\alpha$ and $\beta$, respectively, you have

$$
(i_*,j_*) (\alpha, 0) = (i_*,j_*) [a] = ([i\circ a],[j\circ a]) = (\alpha, \alpha) = (\alpha , \beta)
$$

and analogously for $\beta$.

EDIT2. More details. To break a long exact sequence

$$
\dots \stackrel{f_{i-2}}{\longrightarrow} A_{i-1} \stackrel{f_{i-1}}{\longrightarrow} A_i \stackrel{f_i}{\longrightarrow} A_{i+1} \stackrel{f_{i+1}}{\longrightarrow} \dots
$$

in "the standard way" means that, at any point, you can get short exact sequences like

$$
0 \longrightarrow \mathrm{ker}\ f_i \longrightarrow A_i \longrightarrow \mathrm{im}\ f_i \longrightarrow 0
$$

where the arrow on the left is the inclusion and that on the right just $f_i$.
