I don't see why not. A booking is a booking. Just check yourself in when in LA for both flights. But who knows. I am sure someone will tell me I am wrong. I will say, though, that in the past I have checked in with different PNRs and different tickets at the same time as if they were all connected. I believe that BA can tie the bookings together as well but I am not sure. A PNR isn't a magical being and there should be no effect of having two versus having one as long as they are connected which I would expect that issuing boarding passes would do. I also would be less concerned considering the flights involved. Without a major delay (which you could ask at the airport to be protected against), there is enough airtime to make up most delays.