Doubt From Center of Mass

The max height reached by com should be L according to me

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

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Thanks @Shwetanshu_2018 and @Tanmay_1 Sir for sharing approch