So just put it in "backwards", right? 🙂
Thanks for the super-quick responses everyone.
I just got a new batter for the Odyssey, and the Caravan (which hardly gets driven) could use a new one itself -- but I'm pretty confident the Odyssey pull is in more than decent enough shape for what use the Caravan gets. It's WAY newer too than the one that's in there now.
Though as I type that now, it seems I'm thinking about this backwards... if it's used LESS then the weaker battery will be worse for the Caravan as it sits longer periods of time between being driven.... dang it!