A gram of each is on its way to my hands but I suddenly find myself wondering, is there a cross-tolerance factor?
If I develop a tolerance for one will it make the other one less enjoyable?

well im guessing that your probably correct with the cross tolerance thing.
so just doing it in moderation should be fine. it sounds like your getting a pretty small amount, but i dont know like an 'average' methylone or butylone dose is. just keep every dose spread at by like 2 days and if you notice your not getting as high then make it 3 or 4 days.