Which one should I get and do they make dual tenders or do I even need one?

I'm a fan of schumacker.

ok... i felt the need to put my .02... didnt have time to read all posts..  but i use a shumaker also have little quick connect installed on my batteries... and i have hooked it up to the passenger side battery, assuming both batteries get the charge.  i had something drawing down my batteries so i kept the tender on there while i went away for work for 3 days.  when i came back the tender would be on green but only one battery got the charge.   i know this because i put my battery load tester on the battery ... disconnected prior of course. .. only one battery got the charge.   maybe terminals werent 100% clean ...  but that is just my finding...   my point is, its not as easy as your motorcycle makes it look...  that little battery and battery tender do well for motorcycle batteries.. just not sure for these bigger batteries..   

If only one battery got the charge you have a crossover issue.

As far as the size it shouldn't matter if there is no drain, the batteries are in good shape, and they are charged when you start tending.

I tend to agree with AH64ID... The positive cross over cable between the two batteries must be damaged. Possibly a bad ground cable too. I would do a voltage drop test and see which one is bad.