From the Battery Tender Web Site.... Short version, yes it is OK to charge the same voltage batteries in Parallel....

Can the Battery Tender® Plus battery charger be used to charge more than 1 battery simultaneously if the batteries are connected in parallel?

Yes, but with restrictions. A parallel connection means that positive posts of each battery are electrically connected together and the negative posts of each battery are electrically connected together. The voltage of a parallel connected battery pack is exactly the same as the voltage of each battery in that pack.

If the nominal battery voltages (i.e. 12V, 8V, 6V) are the same on each battery, and if the batteries are the same lead acid type (flooded, AGM, or Gel Cell), then yes, the Battery Tender® Plus battery charger can be used to charge more than 1 battery simultaneously when those batteries are connected in parallel. Just remember that 2 batteries in parallel behave like one large battery. The charge storage capacity of each battery simply adds together. Two 12 volt batteries, each with 25 amp hour capacities, will look like one 12 volt battery with a 50 amp hour capacity. You may be able to charge more than 1 battery simultaneously, but it will take longer to do it.