That would be a huge battery tray in an engine compartment to fit 2 gc-2 batteries. i think a pair of GC-2s would test at about 700 to 800 CCA, so more than enough for most engines.
Diehard AGM platinums used to be relabelled Odyssey, but now they only have the diehard gold AGM which is a Deka intimidator, which is a bare bones AGM with little benefit in CCA over a regular flooded starting battery.
Here is a group 31 starting battery at 950CCA
http://www.oreillyauto.com/site/c/d....oap?ck=Search_N0369_-1_-1&pt=N0369&ppt=C0327
Here the Deka intimidator group 31 is rated at 925 CCA
http://www.remybattery.com/intimidator-9a31-battery.html
So for a purely engine starting duty, CCA figure wise,, the Flooded battery above beats the AGM of the same group size, for about 60% of the price of the AGM.
If deeply cycled, all factors being equal, then the Deka would likely outlast the starting battery, perhaps by a good margin, perhaps not.
The Deka Intimidator is likely the most relabelled/rebranded AGM battery sold in North America. Autoparts stores, Costco, Sam's club, Sears, they all relabel/rebrand and retail the Deka/Eastpenn Intimidator AGM battery.
And it is a OK AGM battery for the price, and it is made in America, but it cannot compare to An Odyssey or Northstar or a Lifeline AGM battery in cranking performance, or deep cycle performance when properly recharged. If it is to be used solely as an engine starting battery and rarely if ever deeply cycled, it will perform well. if it is cycled to 50% state of charge each cycle, and promptly and fully recharged perfectly, they claim it is good for only 350 cycles, where as Odyssey says 800 cycles, Northstar says 900 cycles, and Lifeline says 1100 cycles.
I would consider the Deka intimidator AGM battery to be a good starting battery and a poor deep cycle battery, and a marginal dual purpose battery, unless deeper cycling were infrequent.
XS batteries, I am not familiar with, I though they were marketed to the loud car stereo crowd. I Wonder who makes them and how they stack up to the TPPL AGM batteries of Odyssey and Northstar
The matching battery types for engine and house is largely overhyped too. The vehicles voltage regulator is in control, either battery will accept what it wants at the system voltage allowed by the vehicles voltage regulator, which is almost always too timid, and so far from ideal, that two different batteries sucking what they want at that voltage through the wiring involved, makes little difference.
I alternator charged an AGM and flooded battery in parallel for years, and got 500+ deep cycles from the flooded, and now am using the same AGM battery for both house and Engine duties, and it is performing very well at ~450 deep cycles.
During alternator charging, AGM engine and flooded house, or visa versa is OK, but these two types should NOT be left to discharge in parallel as the AGM will be worked much harder as it has less resistance and a higher resting voltage. When the engine is running, system voltage is higher than either battery's resting voltage, so one battery cannot feed the other.
So as long as the isolation method chosen separates the batteries on engine shut down, there is no problem in having a flooded battery for starting and an AGM four house duty, or visa versa. The only real time where having both batteries be the same would be better, in this application, is if the isolating device fails and keeps both house and engine batteries in parallel the whole time and the driver/owner is unaware this has occurred.
Even then it would take some time for Unideal to reveal itself, and that time might not be any/much shorter than if both batteries were the same and discharging together in parallel.
It is really hard to beat a pair of gc-2 6v golf cart batteries as deep cycle batteries. Their price and performance, is pretty much impossible to beat. I recommended AGM for this minivan simply for the lack of gassing when charging, and not having to worry about spilling, or properly venting of flooded batteries. I also assumed its location to be within the passenger compartment.
But the offgassing issues always have people saying they have no issues with it, and others who can't stand the stink of a charging battery, and we will beat our chests yelling our opinions from the roofs of our vans in order to get someone to agree to what is a personal preference and has no right answer applicable to all.
And besides, my opinions are facts, and inarguable
