Exactly. And remain non-commital, don't let one "own" you through their body language. Don't be afraid to play with the other's hair if one is leaning on your shoulder or something. Be assertive both verbally and in your body language. You want both. Simple. The biggest mistake here would be being afraid of "offending" them. And when they are taken aback (and they will), assert yourself again.
Keep in mind, I'm offering you advice, and I've never been in a threesome. But it can happen. It's not beyond you.
Seducing two women simultaneously is not very different than seducing one. That I can tell you, mojo. Good luck, but if it doesn't happen, it's still a good time trying.