Connect them in parallel, When they die (Which can be anywhere from 1 to 10 years from now) replace them (If you have the headroom) with a pair of GC-2 Golf Car batteries wired in series,, These are TALLER than the Group 24 but should fit in the same tray otherwise.
But since you now have the G-24's Use 'em.
Paralling older and newer batteries.. Many will warn against that but I've been doing it for years, there is a very slight danger,, but any time you put two batteries even consecutive serial numbers in parallel that danger exists and it remains the same. So do not worry about it.
(The danger,, one of 'em has a couple of cells short and "Explodes" spraying the battery compartment with acid... Strange thing is the odds of the new battery failing in this manner are almost exactly the same as the old one doing it, in fact I had a battery fail in this manner (it was not in parallel so it did NOT explode) in about 1 week from installation, (Thankfuly it had a lifetime warranty)... The "explosion" happens because if 2 cells short you now have a 12 volt and an 8 volt battery in parallel, the 12 tries to charge the 8, way faster than it should charge, so it goes POP.