Cut off the weaker of the two branches and any that grow to the inside.

Here is a link that might be useful: Propagating Perennials

Wait for it to grow, and see what it looks like at the end of the year. Any branches that are in the way, or obviously crossing/rubbing another branch can be removed at any time. The more leaves it has, the better root system it will grow, so I would leave it until next spring.

I think you will be pleasantly surprised at how well a one-year whip can grow into a nicely branched tree. Let it "do its thing" and if you don't like it, come back for more advice. If you can post a photo, than you might get other advice, but without seeing it, that's what I would do.