Yes, charging is a bit of a pig, cos putting a diode in
just for the other battery, means it wont get a proper
charge.
Thats why ive suggested splitting the existing diode pack
this means that the auxillary battery will get the same
treatment as the duty battery.
Except of course that the 'sensor' or regulator is on the
duty battery, so i suggest swapping that occasionally.
As to using both when starting the vehicle,
after some thought i would suggest just fitting two
starter solenoids, one for each battery, that way they
would only be joined for starting.
Both solenoid coils being wired together.
Best of luck with it,
John