While there are all sorts of clever things to do with these devices, I believe the main purposes of them was to allow the two main characters to easily teleport *to* each other (rather than away from each other) - meaning both of you keep one in your inventory.
In the original games the pyramids where intended as a means for "fast travel", for example from a dungeon you're exploring to a town with a vendor to sell your loot.
In Original Sin, I'm mainly using them this way. When I need to unload some loot, I simply drop a pyramid, then use the "normal" portals to return to town, then sell the loot and stock up on consumables, and finally use the second pyramid to return to the location I was previously at.
To be honest, I hadn't even thought about using them the way you describe. I haven't had any need to make my chars go to completely different location at once. Though as soon as I finish my first playthrough I'm teaming up with a friend for coop, so this trick will come in handy then.