Sounds like it 50/50 on whether it was legal or not.

It's technically possible that they pulled the trip coverage (TC) for the first trip because that was the only one they needed to cover at that time, so they only needed one pilot and it went to the junior pilot. Within minutes, the 2nd trip was uncovered, new TC was pulled, your name is now on it so you get the second trip.

Now if both were known about at the same time, then both trips should have been on the same TC....in this case your name should have been on it, you were "needed to fly", and you preference for the first trip should have been honored.

I've had this happen numerous time and a call to scheduler gets its corrected.