His overall influence compared to others is the same, but his influence on the top 20 is now smaller, either because he can only define the top 10, or because his vote will only be x/2 compared to the people who don't split their stack.

There is no but, it is really simple but it obviously still evades your grasp:

Xmv = x% of all v= x% influence over the top 30 witnesses.