That's been my experience, too. Couldn't combine, even when on the same airline.

Anecdotally, folks will say the airlines could/might protect you in the event of IRROPS, but I have never seen anyone actually post that this happened. I've done my share of nail-biting about this a couple times during MR or other odd bookings.

I have seen accounts of folks who booked two separate tickets, different airlines, same alliance. Missed an outbound connection and the airline they were connecting to hung them out to dry.