Currently in order to vote in any witnesses small stake has to overcome the full weight of all stake voting for the 20+ witnesses acceptable to the majority stake, so something like >50%.

If votes have to be split up then in a minority attack situation, the majority splits its votes across 20+ "good guys" and the minority need only achieve 1/20 (5%) of the majority voting power to vote in one malicious "bad guy" witness (not necessarily all that bad, but strictly speaking undesirable) and 7/21 (33%) to vote in enough to break BFT.

It might be good to resplit the governance and block production roles as was the case in Bitshares, each with slightly different rules. The governance roles might favor broader consensus while the block production roles favor tighter chain security. But I haven't thought this through sufficiently; it could open up other attacks or undesirable outcomes.

There are probably other undesirable effects to different voting systems apart from security too. If voting for another witness splits your votes then up-and-coming witnesses will have a much harder time getting votes at all Most voters will choose to either support one of the top 20 or maybe 21 or 22 trying to push them in. "Wasting votes" on #56 won't happen.