I just put this 5 year old dawn redwood into her bonsai pot. Not pruning or wiring until next winter. I have never seen a redwood with a split trunk, see picture. Picture is of the front, trunk on right side in question. Suggestions? Thinking about keeping it, but not sure.

I would leave it in the ground for a few more years. It is a very young plant still and will need some time to thicken the trunk

It is not native to my area, so I can not put it back in the ground.

I would say yes put it in the ground, however, you said it isn’t native... do you mean that it’s too cold or hot for it to grow outdoors? If you need to keep it indoors make sure it gets as much light as possible, also thicken the trunk by finding a handful of progressively larger nursery pots and potting it up when it gets pot bound. As for your question regarding pruning you have a lot of interesting options, you could prune it back to a single trunk with just the trunk on the right and have good movement you could also prune the strong tip of the branch the the right that is straight back to that smallish branch moving to the right just above the two branches going sideways or prune it back to the short stocky pruned branch and go with the straight trunk as primary, really a lot of options here though I would suggest doing something in regards to the two trunks this winter so that they can develop different sizes rather then be the same size throughout, you could also keep 1 as a sacrifice branch for a bit but make some choices on the two trunks or else you might have issues later

I am in zone 6A/B in Southeastern Connecticut, USA. Dawn Redwoods cannot withstand frost. The temperature in winter, just coming out of it and starting spring, can go below 20 degrees F.

a quick google search found this listing on arborday that says Dawn Redwood is hardy to zone 5 further the listing on evergreengardenworks says -20 degrees for cold hardiness here . So I would assume it's safe to grow it where you live outside year round. I would still suggest proper winter protection as you would for any potted tree some mulch around the pot or assuming it goes leafless stored in an unheated garage would be suitable further it should be perfectly fine in the the ground with just a simple mulch to grow out. I would not, however, bring it indoors at all though.