Natural frequency: Two objects with a spring

Question:
Two objects which have equal masses, $m$, are connected with a spring whose force constant is $k$. The objects are placed on a frictionless surface and oscillating without external forces. Find the natural frequency of this motion.

Answer:
$x_1$ and $x_2$ are the displacements of each object from the natural length. The displacement, $x$, is the one for the spring, so $x=x_1-x_2$ for object 1 and $-x=x_2-x_1$ for object 2. Set up the equations of motion:
\begin{eqnarray}
mx_1'' &=& -kx  \\
mx_2'' &=& kx
\end{eqnarray}
Subtract (2) from (1).
\begin{equation}
m(x_1''-x_2'') = -2kx
\end{equation}
Since $x'' = x_1''-x_2''$, we have
\begin{equation}
mx'' + 2kx = 0 \quad \rightarrow \quad x'' + \frac{2k}{m} = 0
\end{equation}
The solution of this differential equation is
\begin{equation}
x = A\sin\left(\sqrt{\frac{2k}{m}} t\right) + B\cos\left(\sqrt{\frac{2k}{m}} t\right)
\end{equation}
where $\sqrt{\frac{2k}{m}}$ is the natural frequency of this spring motion.

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