  Let $f$ be a function on ${\Bbb R}^2$ in the inhomogeneous Besov space
$B_{\infty,1}^1({\Bbb R}^2)$. For a pair $(A,B)$ of not necessarily bounded and
not necessarily commuting self-adjoint operators, we define the function
$f(A,B)$ of $A$ and $B$ as a densely defined linear operator. We show that if
$1\le p\le2$, $(A_1,B_1)$ and $(A_2,B_2)$ are pairs of not necessarily bounded
and not necessarily commuting self-adjoint operators such that both $A_1-A_2$
and $B_1-B_2$ belong to the Schatten--von Neumann class $\boldsymbol{S}_p$ and
$f$ is in the above inhomogeneous Besov space, then the following Lipschitz
type estimate holds: $$ \|f(A_1,B_1)-f(A_2,B_2)\|_{\boldsymbol{S}_p}
\le\operatorname{const}\max\big\{\|A_1-A_2\|_{\boldsymbol{S}_p},\|B_1-B_2\|_{\boldsymbol{S}_p}\big\}.
$$
