Hey Seahorse, you can share one trans between two hds units if you have the units linked with a ethernet cable, if you want to use the structure mapping, I would definatly say to get two units or one 8 or 10, because you will need two sd card slots to record the structure maps and hold your mapping card, I'm looking at getting a second HDS7 now so that I can use that option. (just to confuse you a bit more)