Based on an old post, I’d say that stacking the two multiballs is odd but not a malfunction:
During the stacked multiball I can understand why it would be possible to get multiple add a balls. Both T-Rex and Raptor allow you to add a ball at the solid blue Supply Drop target. With the two multiballs stacked maybe the first collect of a Supply Drop uses up the “one add a ball for T-Rex” and the next collect of a Supply Drop uses up the “one add a ball for Raptor”. But I haven’t tested this myself to see if it always gives up to two add a balls or to see if it lets you get more than two.
Also, if they make it to the third phase of Raptor multiball then they get all 6 balls added at once.