Originally Posted by ijgordon

I think if you book the two human passengers on one reservation, and then call up to add an EXST, it will go in a separate reservation cross-referenced. I don't *think* that should inhibit the upgrade on the original PNR. You could abandon the EXST if you both cleared, but you'd have to try and make sure that any continuing travel on that reservation isn't cancelled. It could be a little messy. But as others have said, the chances of upgrades on a 319 are low, and even if you both cleared, you'd probably won't be able to sit together.
Personally, I would save the $ and take 3 coach seats.