Your past and near future electronics set-up is similar to mine. I had a LMS-332 that was later replaced by a LCX-27. The other unit was a 997. Sharing waypoints between a Humminbird and a Lowrance was doable but was a PITA. I kept my 997 and went all Humminbird when I upgraded boats. You will like how much easier it is to share waypoints between units.
I would keep the 997 on the console with its SI transducer on the transom and put the 798 on the bow with its SI transducer strapped to the TM. Use the SI transducer on the TM for 2D and leave the universal sonar alone. Mount a GPS receiver near each transducer for both units for the best accuracy. Consider Interlink to make sharing waypoints really easy. I press the "Mark" button on one unit and the waypoint shows up on both screens.
I'll holler at you next time I'm in your neck of the woods.
Keith
2008 NWR Bash Crappie Champion
2010 NWR Bash Yellow Perch Champion
2010 Buggs Bash Smallest Crappie Award