Personally, I don't think these two airlines are well matched and I agree with other posters who suggested that there are better options for both airlines. That said, let's look at how it might work out, fleetwise.
As far as I can see, the only commonality between them is on 757 (both PW
powered) and 763ER (some DL
powered). The 777s have different engines, although I guess that's surmountable, if they both stick to their own bases. In time, I guess they can bring the fleets closer together, but with such a huge fleet, that would take until the middle of the next decade. They'd both want to stick with a single fleet.
Both are likely customers for the 787; a combination of -8s, -9s and -10s could replace the 763s, 764s and 777s in due course, although UAL would probably want something a little larger; the deal would have to be approved in Europe and while nothing would be said officially, I would be surprised if it weren't hinted that an Airbus order - somewhere along the line - might "incentivise" approval or "accelerate the process".
Midsize, yes, commonality on 757s - a total fleet of about 150, give or take, but these aircraft aren't getting any younger. For the moment, at least there is some commonality. Given fuel costs and the increasing pressure on airport slots, I expect the trend to be towards larger models, so UniDelta would probably be at the top of the queue for an aircraft in the 757's size category, maybe some 787-3s for domestic use too?
The smaller types ... that's where they divide quite significantly; no commonality: 733/735s and A32xs at UA
and 737-800s and MD88s (plus 73Gs to come). Really, the most likely outcome, I would think, would be for both carriers to get on board with Boeing and help develop the 737 replacement; Boeing has indicated splitting the new model into two sizes, the second being aircraft of the 150-200 seat size (presumably in various models); I have an inkling, without any foundation, that Boeing and Bombardier will come to some agreement on the new C-series aircraft.
Workable from the fleet perspective, over a long period of time, but as said above, both airlines probably have better options. At least the ball is rolling anyway ...