I'm having trouble getting the PHL-YUL nonstop that I want to have miles availability, and the best alternative is a one-stop via LGA, PHL-LGA on US; LGA-YUL on AA. I'm booking these with BA Avios, and they consider US and AA to be separate partners, even though they have the same parent company. To book a multi-partner ticket, it's 30,000 miles, to do it this way (separate reservations) is 13,500 miles.

My question is would US/AA be willing to combine the two tickets (assuming the name on both is the same, connection is legitimate, etc.) after purchasing it, or would I be risking a misconnect and getting stranded in LGA?