At best user B could vote 10-20, but then user A would still control 1-10 and 21-30, and the point is still that the stake controls the witnesses, not the number of votes, and undoubtedly it will encourage stake splitting and since the witnesses aren't controlled by any one party there's no reason to encourage stake splitting to circumvent the limit, we might as well set it on one witness vote, why stop at 10, it will have the same effect: people will split their stake up to consolidate their power into more meaningful ways.

https://steemit.com/steem/@ura-soul/visualising-the-votes-for-the-top-50-witnesses-with-and-without-the-freedom-pumpkin-votes