Using one site seems easier so I'd say do that.

There are those who lack access to Furaffinity on Twitter and vice-versa, keep them both.

I don't have twitter, so I've only ever handled it through FA comments. You could either handle it in rounds, with each comment counted as either a vote or a raffle ticket, or you can handle it all at once with each change being first-come/first-serve (which is how I handled my last Build-a-Chimera event: https://www.furaffinity.net/journal/10094442/ ). Depends on how you'd like the event to progress.