I find that actors like to be tested (okay, they don't really - but one can sell it to them this way

So having doubling up is a good way to push an actor into having to do a little more work than usual - which is always a good thing.
Had one fairly heavily produced piece with actors on all fours for the duration, requiring kneepads and gloves - and another piece with actors hanging from scaffolds - so I like yanking actors out of their comfort zones.
But back on topic - I think as a writer, for doubling/tripling up, just make sure that there's sufficient time, dialog-wise, for the actor to get off stage, change, and return (as the new character).
Because, as you probably know, if something can go wrong, it probably will.. so give thought to how much of a costume change you're requiring of someone, and make sure they've got 'just' enough time to do it..
Another thought, given how much it sucks for actors to be sitting around backstage, with only one or two entrances - I think the more 'work' and on stage time you can hand an actor, especially secondary role actors, the more fun they'll be having..