  Let $\mathcal{SS}_k(n)$ be the family of {\it shuffle squares} in $[k]^{2n}$,
words that can be partitioned into two disjoint identical subsequences. Let
$\mathcal{RSS}_k(n)$ be the family of {\it reverse shuffle squares} in
$[k]^{2n}$, words that can be partitioned into two disjoint subsequences which
are reverses of each other. Henshall, Rampersad, and Shallit conjectured
asymptotic formulas for the sizes of $\mathcal{SS}_k(n)$ and
$\mathcal{RSS}_k(n)$ based on numerical evidence. We prove that
  \[
  \lvert \mathcal{SS}_k(n)
\rvert=\dfrac{1}{n+1}\dbinom{2n}{n}k^n-\dbinom{2n-1}{n+1}k^{n-1}+O_n(k^{n-2}),
  \]
  confirming their conjecture for $\mathcal{SS}_k(n)$. We also prove a similar
asymptotic formula for reverse shuffle squares that disproves their conjecture
for $\lvert \mathcal{RSS}_k(n) \rvert$. As these asymptotic formulas are
vacuously true when the alphabet size is small, we study the binary case
separately and prove that $|\mathcal{SS}_2(n)| \ge \binom{2n}{n}$.
