If two projectiles are projected from ground with speeds $u_1$ and $u_2$ at angles ${\alpha }_1$, ${\alpha }_2$  with horizontal then relative acceleration between them is (in m/$se{c}^2$)

$\begin{array}{l}{a}_{1x}=0 = acceleration along x-axis; a_{1y}=-g =acceleration along y-axis\\ a_{2x}=0 =acceleration along x-axis; a_{2y}=-g =acceleration along y-axis\\ \therefore \overrightarrow{a_{12}}=\overrightarrow{a_1}-\overrightarrow{a_2}=\left(a_{1x}\widehat{i}+a_{1y}\widehat{j}\right)-\left(a_{2x}\widehat{i}+a_{2y}\widehat{j}\right)\\ \overrightarrow{a_{12}}=(0\widehat{i}-g\widehat{j})-(0\widehat{i}-g\widehat{j})=0\end{array}$