I never actaully saw inside one to see how they are made. I suspect something simple like just splitting the power feed cable into two lines each with a blocking diode so neither battery knows the other is there. But then I question if that's it then how is the charging from the altenater regulated? It would seem to me off hand with a blocking diode type setup a charging source like altenator regulater would not know if the battery was at 11V or 13V since the feed back is blocked.

My question though is with all the fancy gizmo's for charging that supposedly read a batteries condition and adjusts for it what will happen with blocking diodes in the line. Surely something like a smart charger would not even turn on. Unless the charger is only reading it's own output somehow to determine how much the battery will take.
I have both a smart charger (vector) and a booster (maybe vector) and neither will do anything unless connected to a battery with some juice in it. Totally dead battery these things won't even start up. I think a blocking diode here would have the effect of not letting them startup as they would not see the battery at all.

I don't know how a trace controller controls the charging or anything else it may do like dumploads when batteries are charged, so I don't know if diodes or one of those isolaters would actually work with it correctly.

If charging is not a problem using the above, then I would think using one cable into the battery bank for charging and a seperate cable out to the inverter also with blocking diodes would alough both banks to be used on one inverter at the same time.
The cables from the battery bank to the inverter being only one direction, power out, the batteries again should not see each other, but the inverter will be drawing from both banks at the same time and if one bank goes dead then it just draws all the power it needs from the one still having power.

Put some thought into it, somethimes the answers just pop right into your head from no-where. As I was posting this I realised I was planning to something very dumb myself! Hows this for dumbness??

I have been charging a bank at home from the grid, taking to a remote house and running a small inverter in the truck to power a charger to charge those. This works well for now, but I am playing catch up all the time because the big inverter is running from the ones I am trying to charge at the same time. So I was planning to set up heavy cables (don't have long/heavy enough ones yet) and such to run the large remote inverter directly off the truck bank and disconnect the remote house bank and use the small inverter and charger to charge those also from the trucks bank. This way I am not using power from them at the house as I am charging them. They will get a fuller charge and I can more fully drain the ones on the truck. Sounds good?

 DUMB DUMB, I just realised!!
I would be running a 5,000watt inverter at about 500-1000watts or so, something small anyway most of the time. The small inverter only powers my charger up to 20 amps, it's a 40amp charger though. Ya, duh on me! Why not just forget the small inverter on the truck, why run 2 inverters? One is barely being used the other maxed out and still not enough! Just run the 5K inverter off the truck, battery charger from that, charge the remote bank at the full 40 amps!! 1 inverter running a heavier load and 40amp charging is much better than runnning 2 inverters and only getting 20amp charging!!
 I only do it because I need power there and no way to make it near by, I don't own that place so limited to what I can build or change or install (working on it though). Nothing is really great, but boy was I planning something really dumb as for the 2 inverters!!!

Anyway I was going to look for blocking diodes myself and see what I need for the power I would be drawing at that location so I could have both banks wired to the inverter, neither knowing the other was there, then just disconnect whichever set needed disconnected at the time. Sort of a oneway wire, power into the inverter from both/either banks but any wire would be dead when disconnected from a battery bank. No back feed through it from the other bank. Eliminate some manual switches and chances for error this way.
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nothing to lose

I agree with Ghurd here. In an ideal situation you would use batteries of the same type, size and age but if the opportunity arises to put together an odd combination on the cheap it will probably work fine for the cost involved.

An oddball combination of batteries will work a LOT better than no batteries. Simply keep an eye on them to be sure they play well together.

Cheers.

TomW

The Truth is the Truth, even if no one believes it; and a lie is a lie even if everyone believes it