Can I charge multiple SLA batteries at once like this?
That should work, if the charger is designed to charge lead-acid batteries, and can deliver a reasonable current relative to the capacity of the batteries.  A 1 or 2 Amp charger would probably be OK with up to 24 Ah of batteries, but may not be of much use with 100 Ah.  

A simple 12 volt DC power supply wouldn't work.