I'm looking at ways to fly from AMS to BOS, and one of the options is:

BA AMS-LHR leave 4:10PM arrive 4:30PM
UA LHR-BOS leave 6:00PM arrive 8:40PM

When I search in Concur, this comes up as a D fare on BA and a J fare on UA, each with its own ticket (on the respective carrier), but booked on a single reservation by a corporate travel agency.

When multiple tickets are combined on a single reservation like this, does anything change about misconnect risk? If BA is late to LHR and I miss the UA flight, is BA still obligated to get me to BOS, or will it be up to the travel agency to make any changes? I would have guessed that this works normally (the carrier causing the misconnect is responsible for rebooking), but I'm not sure if BA would claim that they completed the obligation under their ticket and have no responsibility for travel onward past LHR.

I don't think I would have had this question if it were a 016 ticket for the whole itinerary, but at first glance (based on Google Flights and ITA Matrix searches) it doesn't seem possible to combine BA business and UA business on a single fare, though it is possible in economy (UA fare YFF77WW) and premium economy (UA fare OFF77WW). Looking at fare rules and MPM in ExpertFlyer, I don't see why this wouldn't work with UA fare JFF77WW. (If anyone has ideas for this, let me know! Both the BA and UA flights appear to have availability in all business class booking codes, and I can't tell what's preventing them from being combined.)

Thanks!