Given two object of mass $m$, the gravitational acceleration experienced by each is given by

$ \displaystyle \frac{\mathrm d^2r}{\mathrm dt^2}\ =\ -\frac{Gm}{r^2}$

(Note the minus sign: it is because the acceleration increases as the separation between the objects diminishes.) Now $r$ is a function of $t$; if you can express $t$ as a function of $r$, you can substitute it into the RHS and analyse how acceleration varies with time.