Hey Pickster!
I haven't seen any VERY bad experiences. Meaning if 2, 12V are connected in parallel something very bad happens.
I have seen a bad battery drain the others, fast or slow(1), keep the others from charging(2), interfere with the others from suppling amps at a decent voltage(3), etc.
(1) If one battery 'leaks', as its V drops, it is sucking power from the others to stay charged. Like if one battery leaks 1A at 12.4V, the other battery can't get past 12.4V if the charge amps do not get past 1A.
(2) Some batteries seem to 'leak' a lot faster as the voltage increases, meaning the 2nd battery never gets up to regulation because the leaky battery takes all the charging amps. Like one battery leaks 1/4A at 12.4V, but it leaks 5A at 12.45V. The good battery can not get higher than the bad battery will let it even at high amps.
(3) Related to #2? As the better battery supplies the majority of the amps, the 2nd battery volts drop fast and it starts using part of the power to charge itself. (?) Very strange but I have seen it. No guess at an explaination for that.
It is best to have the batteries at about the same age.
Probably very important to have them at the same voltage, when they are first connected.
I have seen batteries that should never have been connected... connected!
Many worked fine together for years. Others did not. That leads me to currently believe if the charging system can keep up, it should be OK if you keep an eye on them individually.
You gotta do what you gotta do.
G-