First of all, if you have a wet cell then it is not maintenance free. If it is maintenance free, it is a AGM or Gel cell.
If you are charging the battery types in parallel, be sure they take the same charge voltage, it may be on the battery itself or you may have to look it up online. If they take the same voltage, and the same temperature compensation slope, you can charge them in parallel, if not, then you are probably doing damage. You may be overcharging one type and causing it to vent and dry out, or undercharging one type causing excess sulfation, or both.
Re Cruisingdads comments, if you are using a portable charger, i.e. Wallyworld special for automotive use, STOP. They are OK for starting batteries only. You need to get a good quality constant voltage type charger.
SOC and voltage measurements are very dependent on temperature, and not that accurate anyway. A fully charged battery may have a open cell voltage of 2.5 volts if cold enough. The best way to determine state of charger (SOC) is to monitor the charge current into a battery. When it has leveled off to a few 100s milliamps, depending on the capacity of the battery bank, and unchanging, then you can say it is fully charged. Monitoring amp-hours out vs. amp-hours in (multiplied by the efficiency, about 95%) is another good way, Iíve seen some marine chargers that do it this way.
There are two types of fools...
One says this is old, and therefore good..
The other says this is new, and therefore better...