if your tender puts out somewhere in the 13-16 vdc range, then there is no reason it can't tend more than one battery. it will do so in less than effective manner. that is the battery should not be dead or near dead.
and you want to connect the 2 batteries in parallel. positive lead from the tender to the positive battery terminals, and negative to negatives.
if connected in series will half the voltage going to the batteries and not provide any charge.