It is really easy.

Right now, his 30 votes, can basically vote all the top 30 witnesses => Very strong influence!!!
If he only has 10 votes, he can only vote the top 10 witnesses => 50% of the top 20, much better.

If he splits it into 2x10 votes, each of these votes is worth only half. (Yes it is still worth a lot, but only worth half than what it'd be if he hadn't split it)
So, if the orcas and dolphins don't split their stack, each of their vote is worth more now after the change (because they didn't split their stack).

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.