I don’t see why you would have to update both units at all. It is true that the new SW screws with the LakeMaster chips, not sure about Navionics though. I have 3 HDS units in my boat, two of which are networked using the Ethernet cable and only ONE is running version 4.0. The other unit is running 3.0 I believe. I haven’t had any issues with them not being on the same version.
If you desire is to share a mapchip from one unit to the other THEN I think you have no choice but to update, but if that map chip is a LakeMaster (and possibly Navionics) then I think you might have trouble.
IMO the best solution is to go with the Lowrance Lake Insight Pro chips. I know its another cash outlay that you probably don’t want, but it will be inevitable that you will need to get one at some point seeing what Lowrance has done here with this recent update.
The Lowrance Lake Insight chips use the same data from the LakeMaster chips PLUS they add in Lake of the Woods AND Rainy River on the one chip where that was two separate chips with LakeMaster.