Absolutely. In fact, just before I got hired, there were guys who were in what was called the "Dual Pool", they were qualed on two airplanes, like DC9 F/O and DC 8 engineer, and they were on reserve, for Both! Scheds could/would call you out for either one! If you were getting close on landing currency, you called scheds and they'd get you on a trip to get your landings. I knew guys who were both 737 F/O's and 727 F/E's.

When I went to MD-88 Capt. (1992) they trained me (at 757 F/O rates of course, because I wasn't Converted yet) and then gave me two IOE's. Then they sent me back to right seat on the 757 where I waited to be converted...for 3 months!

And then, the month before I was to convert to 88 Capt. they made me go to 757 RECURRENT!! But I had vacation in that month, and all they would have had to do is drop one 757 trip, to NOT send me to recurrent, OR convert me one month earlier, and not send me to recurrent.

I called everyone from the CPO to DALPA to try to get out of going back to recurrent, no dice. Luckily my 757 IP was very understanding and let me slide through the recurrent with minimal questions for me.

"Tim, How many engines on the 757?"

Me: "Um...three...NO TWO!"

It was in C2K I think, that we finaly got that changed, so now you don't ever go back to your previous airplane, after you complete training, they convert you right away, if not before you've even started training.