I think the retarding force on top mass is its own weight plus tension of thread. The thread has tension T=mg
So i think net downward force =mg+T=2mg
So retardation a=2g
When the 2nd ball is still resting on the surface then Tension continuously changes and and it’s value won’t be 2g so I don’t think this method will give correct answer !
I think coincidently the numerical values came out to be same
Please ignore post#2 using that method answer is coming out to be correct but 1st step is incorrect
Correct methods is