You can use your Vagabond to recharge one battery and invest in a Battery Tender or similar smart charger that only puts a half amp maximum (very precisely) into the battery during recharging to recharge the other at the same time. That will get you back in business in the minimum time.
Realistically both batteries probably will need to be replaced together in a slightly longer time versus use one battery, kill it, get another battery, use it until it is dead too. The benefit of using two batteries is the convienence of not looking like an idiot during your paid shoot when one or all of your lights stop working and you have to troubleshoot to see if the tube burned out or if the battery ran out. Nobody needs that hassle.
I appologize if it seems I am lecturing, but I have dealt with SLA, lithium and most other battery techology issues for the past 5 years or so dealing with custom one-off and/or limited run led and hid
led flashlights. Mr. Buff has done a remarkable job with these Vagabonds mixing cost versus performance. I can tell you that using any other battery system other than what he has would have raised costs really fast and honestly, not made the user's life any better.
Bob E.
It is quite clear to me that you have a vast knowledge about batteries which is really appreciable. But I want to highlight the point of your apologizing. Really you are brave person !