Yeah, double slotting seems like it would lead to some interesting merged effects, though I'm not sure how all of those could go. One's that share some common traits like the plane that make water everywhere and the plane that makes water burn make sense (even if that would be a terrible idea to use), but the others... and like you said, that's before considering even more icons being put in the same spots.
I have noticed a fair gap in undeath based ones. There's one plane that boost specifically incorporeal undead over their corporeal counterparts, with the plane that can fit in any slot able to destroy all undead with sunlight.
Thanks, those are some handy docs to use, especially if my PCs feel inspired by parts of the portals the Ob came up with during the convocation. I took a closer read at 12, and wow, there are a fair number of mages that have ended up screwing over one plane or another, huh? Also Drozani is still terrible, but looking at it's description in Adventure 12, it seems to be that way because of the Sea God in Hunlow. Hunlow loses any of it's traits if the god is killed, but I wonder if killing it would just change how things are on Drozani.