They can check your bag through to UA on two tickets provided the connection meets or exceeds the minimum connection time in sea. It's easy to confirm, try building that itinerary on any OTA like Expedia. If you can find it for sale, it has passed the MCT test.
You don't mention any return, but UA and AA no longer offer to interline bags on multiple tickets when checking in with them. There could be a few alliance based exceptions but that wouldn't apply with your carriers.