They're separate reservations, so I wasn't thinking about short checking, but the pros and cons of asking LH to through-check on to the separate UA reservation, given the fairly unique IAD setup where immigration/bags/customs happen in different terminals based on whether the bags are tagged for IAD or an onward destination. At most other airports, there wouldn't be much downside to through checking since I could just claim my bags, ckear customs, and then exit to the city rather than drop off the bag if I no longer could make the onward connection and would move on to my backup option. I wasn't sure about the IAD midfield setup since it's been a few years that I've used it, and I never did so on separate reservations.