Amazing theory except the E31 comes factory with 2 batteries. Problem is that the E31 drains so much upon being parked that the need to charge within a month is required whereas the E34 does not. Batts are matched and "at least" 5 years old so that part holds no grounds on my personal vehicle.

whatever you do do not just wire it up in parallel.
anything large enough to be worth the effort will 'fight' the primary battery.
in essence they will take turns dragging each other down in voltage (or trying to charge up the other one) when the car is off, and you'll kill both of them (although maybe if lucky just the secondary) in short order.

Voltage and amperes are just as stated and will charge as required without any trickery, 2 batteries are better than one in a less then adequate environment.

Been there and even matched twins don;t last more than a couple of years or three, even with daily driving.

see above 5 year claim.

Use an isolator wired to ignition. Then the battery is only connected when the key is on, ie driving, starting, etc. IF you drain the primary and the secondary is large enough you'll get a couple of chances to start the car when you flip the ignition on which connects them together again.
Ignition lead on alarm prewire cars is in the 12 pin alarm connector, and readily accessible.

You do know about circuits and my boy would be proud.

Again this is only useful if you are going to be having a lot of key off accessories ( like the lighter/power port always in use, the BMW phone or the vent timer) running, or plan on running a large load in accessory (large amp draw amplifiers for example).
It is kind of a built in jumper cable. it isn't for more key off time.

Agree the vent timer is a big drain but if not used in excess an additional battery should simply not be required in said vehicle.