The term independent of x in the expansion of (1+x)n(1+1x)n is :
(1+x)n(1+1x)n=(1+x)2nxn
Term independent of x in (1+x)n(1+1x)n is coefficient of xn in (1+x)2n
= 2nCn
But we know that C20+C21+C22+............+C2n=2nCn