is there a positive to a shared negative?

[captianjuan]

Im going over my house bank wiring to prepare for connecting it to my 1200h lithium bank. Im keeping my lead acid solely for starting and windless. Ive gone over everything I can access to separate the grounds for my engine gages and alternator wires to keep them part of the lead acid bank, but for some reason im still getting 11v between house - and engine +. no continuity between negatives according to the multimeter. Question is as follows,
is it ok for the two banks to share a ground, and in an old setup such as this, preferable to having it "isolated" with with weird stray current?
is there a concern for some sort of cosssover causing a load on the stray wire and posing a fire hazard?
Do you guys keep a common negative or is one floating or going to a grounding plate?
thanks in advance. always a wealth of knowledge on here.

[smac999]

All of the banks on the boat should have the grounds connected.