If the unit on front is not a gps unit (or "capable" using the other unit's ant.), you will not be able to mark it using gps. I am not sure I understand what you're asking. I am in the same situation you are, or will be. The next unit I buy will the 788ci, but I am just going to have two separate transducers. One on TM, and one on transom. I will move my unit back and forth. So, when fishing I'll mark using the TM ducer, of course, but when I'm running I'll have the maps at the console. It's not that big of a deal to move it. When I'm trolling, I'll generally be up there for a while. When I'm jigging I won't need it, usually.