Wire diodes in a way that current can't flow any way but toward each battery from the charger. (Remember those isolator kits for cars trucks that had 3 posts on a large heat sink? Same deal, and you don't want a circulating current loop between the batts.) Whichever one can use the charge will take it, but they won't be going back & forth between each other this way. (not so good for them)
You can set up a bench charger with a diode on each + of several take-offs or clip sets. They will divide the current according to their state of charge. Chargers' no load vs loaded voltages will vary, so be less concerned with voltage when charging. After any charging cycle give the batteries a 1/2 - 1hr rest before checking voltage. 12.7v NL means they're in good shape.
btw, Just prepped a 'dry-charged' batt for boss' quad. Instructions said to wait 1/2 hr after adding acid before charging and to charge at 2A with a floating circuit charger (14 A/Hr batt, btw) then rest another hour before use. The battery will still work if you rush thru this, but useful life will be shorter. (BTDT with puny 16 A/Hr batts in 'big twin' motorbikes w/o kick start & lucky to ever get two seasons out of one. AGM batts are better when sized small for load and are shipped 'wet')
It's a good idea to trickle any unused battery as all lead-acid types will self-discharge over time, and being on concrete can't be blamed anymore (old myths die hard), nor can greasing/sealing terminals prevent it. I built a YouTuber light bulb 'regenerator/desulfator' (high V, low A pulses, w/1A fuse) and have had good success recovering batts that have no cells shorted internally. No-load voltage reads high on meter's DC scale, but falls right into the mid-13s - 14v when connected.
Battery life is inversely proportional to time spent at full charge, the reason your daily driver's battery lasts so long. Topping up is as important a routine as checking tire pressures, lug nuts/bolts or loader bolts but doesn't ever have to be as much work. tog
