As in both games, released simulataneously that share a storyline. Maybe even involve Ghost Recon?
Each game could be played independantly, but each mission on one leads to the next mission on the other game. That enables some sort of lead or info for the first game and back and forth. You could play each game independantly, or if you own each game, play a mission on one, go to the other to directly continue the storyline, then back to the other after that mission was over if you wanted to do it that way.
Kind of how SC campaign and co-op had a little tie in, occured at the same time, whatever.
Sam Fisher on a mission finds out about a terrorist plot, finishes his mission...Rainbow heads in to take down terrorists, captures one who spills the beans about some terrorist training compound where Ghost Recon happens to be, they go in to take them down...more info comes up and feeds the story further...
Would take a lot of cooperation, a lot of quality control and a really good storyline to pull it off, but it would be nice to have the 3 best titles in gaming working together. Would also be an extra incentive for people to own all 3.