B Elongation of a spring in a two mass system

The Question is posted as a photograph with a. little change. One of the mass in the photo is equal to '2m' rather than 'm'. Now the block won't stop consecutively.
How would we approach the question?

 

Yes Ofcourse as per the question it clearly seems it will.
But please dont say it will be the maximum elongation when force balances cause it will not.

 

sir,
if both the blocks are of equal masses then the maximum compression would. be seen. at the time they both stop momentarily and consecutively the first time..Hence their kinetic enengirs would turn into potential for spring.

But my ques. of what if masses are unequal.
one is m and. other. is 2m.
Idont know how to find the answer then as both masses wont stop consecutively