What he said...
I also do the battery tender. I hook up both batteries on one tender. You go from tender to positive, then from positive to positive. Then for the negative side, you go to the further away negative and then from negative to negative. This is in "parallel but forcing equal charge due to going from one close and one further terminal via the tender.
Basically, if you went to the near terminal from the tender in both cases it could affect the second battery if there was an issue with the first battery.
For me, I normally get 4-years out of my batteries. Granted, I'm from Ohio so the ride time is less and I use the tender in the winter time.