I got this swamp white oak bare root seedling at a tree giveaway a couple years ago, and stuck it in a pot since I'm not ready to plant in the ground yet. But it seems to have developed 2 main leaders, how should I prune to train it for one main trunk?
How to train seedling for main leader?
I think many oak seedlings thrive on being fractious, as your Quercus bicolor demonstrates. Good luck reigning it in.
I would consider tying together the two branches you've shown there. The left one in the picture seems to be growing more, but time will tell. By tying them together, and trying to pull them to a more upright position, that will attempt to convince your young lad to grow a vertical dominant leader.
I would not prune anything off yet. Let the plant show you what it is going to do, and you can always prune later - you just can't put it back.
Nurserymen often grow seedlings in pots like this very closely packed in order to force vertical growth. Your plant has the luxury of no competition whatsoever, so it is happy to spread its wings and gather all the abundant sunlight around it. Great strategy, and set to confound you.
I bet by the time you get a summer flush of growth on this one, you will observe some behavior you like. Let your plant go dormant, hopefully you will decide on its permanent home and put it in the ground, and next spring before the buds break, make your pruning decision. Or not. Another growing season with your encouragement may be worthwhile before exercising your editorial skills.