I'm posting this to get feedback and know its not the best or optimal idea so I expect some flak so let her rip.

Is this dissimilar battery configuration too far out in left field?

I am way under the money it would normally take to do this because of luck and free access to welding cable, heavy duty tinned copper ring terminals, toggle switches and more. The fringe of being an industrial maintenance tech.

During my parasitic drain adventure research I cam upon new Sears Die Hard Platinum 34/78 (Odyssey 34/78-PC1500DT equivalent) with warranty for a price I could not refuse through a company bulk purchase.

This was a panic buy/jump on the great deal now or never again/I thought my battery was defective/ruined.

So I found myself with the Die Hard AGM monster and a perfectly good Deka Ultimate 778MF.

My Brother had an Odyssey Ultimizer OMAX-25A-1B Battery Charger left over after selling his truck and we made a trade for items in both of our basements taking up space.

I already have a Schumacher XC10 maintainer for the Deka.

Waste not want not? My mind racing along sizing up my luck mentioned above in the decent investment savings & resources towards the dual setup I have committed.

I have always wanted to do a dual setup and have pulled the tray parts I need from the local bone yard.

So I am planning to avoid the automatic isolator thing because in the event of long term inactivity (The 2005 only has 72+k on her) they would require different chargers/maintainers.
Example - a Cole Hersee Smart Isolator has no ignition trigger. When the vehicle is off its hot and active. If I wanted to charge or maintain the batteries for long term idleness there would be an issue as well as the parasitic drain. And manual isolation is not a go.

So, I am going the route of a 225 AMP DC Contactor, the very same as Painless uses, and use a 3 way toggle on/off/on illuminated switch.

Position 1 - (White Light) Both connected and charging simultaneously "with ignition on".
Position 2 - (No Light) Contactor open, "main battery only at all times".
Position 3 - (Red Light) Contactor closed, "both connected at all times".

If I have to use a maintainer I select the middle switch and make the truck look like its on life support -

Other than the Die Hard deal, the bone yard battery tray, and the $50.00 contactor, that's all I'll have in her other then the bartering.

Let the camping begin.