Under maximum compression of spring both blocks moves with a velocity of 

Under mass compressed state two blocks move with equal speed.  From law of consumption of linear momentum

${\text{m}}_{\text{1}}{\text{u}}_{\text{1}}+{\text{m}}_{\text{2}}{\text{u}}_{\text{2}}={\text{m}}_{\text{1}}{\text{v}}_{\text{1}}+{\text{m}}_{\text{2}}{\text{v}}_{\text{2}}$

$1\times 2=1v+1v$$\Rightarrow v=1\text{m/sec}$