In theory it is OK to connect them in parallel with two conditions: Each battery must be in a state where it can be voltage charged. This is fine for lead acid batteries unless they are very run down. Very discharged lead-acid batteries have to be charged with fixed current until they get to a minimum voltage, then they can be voltage charged.
Most lead-acid batteries charge at a constant 14 4 volts, so charging several in parallel is really just a charge-current issue. If the charger cannot supply enough current it will likely lower the charge voltage to protect itself.
Charge them separately with a good (3 or more stage) battery charger and see if they hold their charge for a day (setlling at about 12.6 or 12.7 V), or if they charge at all. If they do, you can probably safely charge them together. There are always risks involved when charging lead acid batteries. Keep them well ventilated and fused.
Lead acid batteries will not be properly charged at just 13.8 V. All (not some) lead acid batteries I know need a “bulk” charge voltage over 14 Volts (look up the datasheet of any lead acid battery to confirm this). 13.8 V is just to maintain the charge (“float voltage”).
Power Sonic recommends you select a charger designed for the chemistry of your battery. This means we recommend using a sealed lead acid battery charger, like the the A-C series of SLA chargers from Power Sonic, when charging a sealed lead acid battery. Sealed lead acid batteries may be charged by using any of the following charging techniques:
for secondary (rechargeable) batteries – the stronger battery would charge the weaker one, draining itself and wasting energy. If you connect rechargeable batteries in parallel and one is discharged while the others are charged – the charged batteries will attempt to charge the discharged battery.