I think a DT with former members type of encore would be great for a one off show. Touring like that may be a bit too much and probably doesn't make sense. My general comment about the bands jamming together wasn't meant to be specific about this example.
When Rancid and Dropkick Murphys toured together (they just did this summer, but I'm talking about 3 years ago) they did a joint encore with both bands on stage doing a few covers. It was awesome.
Alice Cooper when touring with Halestorm and Motionless in White had members of both bands on stage for the encore of Schools Out. Also really awesome.
Generally, I think these type of things are really cool. Also, if they aren't doing the joint encore thing the entire tour, sometimes bands do it for the last show of the tour.