Yeah, a higher capacity battery like a 10Ah would be safer for the battery with a 1.5A charger.
Yes, you can charge two together to effectively create a higher capacity 12V battery. If you put a pair 12V batteries in series, you get a 24V battery. If you want to charge the batteries together to divide the current between the two batteries, then you would wire the two batteries in parallel, yielding a 12V battery of twice the rated charge capacity.
However the current will not divide evenly unless the batteries are equally discharged. So I'd be careful doing this if you are discharging the batteries seperately. If one battery is significantly discharged compared to the other, then the charger will continue to generate the charge voltage until the lowest battery catches up. That means that the fuller battery gets the charge voltage longer than when it should have been switched over to float. It might not catch fire, but you battery life will be shortened if routinely over charged.
Go to the SLA battery manufacturer's page and look at the spec sheet for charging recommendations. I know that I ran into some SLA batteries capable of taking a higher charge current, but unless you know for sure, the 20% rule is the thing to use.
Just as an example, most of these commercially available 6V-7.2Ah power systems seem to ship with relabeled Powersonic PSC-61000 A chargers which produce a max charge current of 1A. I bought one from a local electronics supply store for about $41.00.