I am considering replacing my console graph with a Lowrance Elite-9 TI2. I will be keeping the bow graph which is an older Lowrance Elite-7 HDI. Can these 2 units link up for sharing waypoint?

If you install a nema network then you can do a limited sharing. You will have to update the new unit with your waypoints then the waypoints will display on the other unit. There is no syncing but you will see what is there on both units. Lowrance makes a Nema Starter kit to get you started. You may need a extra Nema Tee and 2 ft Drop Cable besides the kit. If you replaced your Elite 7 HDI with another Elite Ti2 you can wireless share waypoints, Sonar, and Map Cards as well as syncing when you add or delete waypoints so you end up with a backup copy of your data at all times through the WIfi. Big sales coming at Cabelas and BP on the Elite Ti2s. Good luck Enjoy

I like the idea of sharing everything, except the sonar. I like my console sonar to be in the back as usual. but I like my front sonar to be under the trolling motor.

But I think I also saw where you can't use map cards. One comes pre-installed and that is the one you have to use. Is this correct?

It has a slot for map card like another Lowrance or Navionics card you can choose the built in map or the external map card you have a choice