Glennster YOU ALREADY KNOW THIS I BET!!!!!!!!
YOUR QUESTIONS: would it work?
ANSWER Sure it will "work" you have 24 volts
YOUR QUESTION: would you have the amp hours from each battery available?
Amp Hours is additive if in paralell NOT if in series. The amp hours you would end up with is basically (subject to other variables of course, never say never) that of the lowest of: Those two 12/s in paralell (which net is the sum of BOTH those batteries Amp Hrs) orrrrrrrr that single 12, and assuming the single 12 is surely less then two 12's in paralell (cuz their amp hrs adds) YOU END UP WITH THE AMP HRS THAT SINGLE 12 ALONE HAS
YOUR QUESTION: any downsides?
When you start mixin n matchin and summing up battery combinations, YOU want them IDENTICAL is size and type and brand and CCA and age and Amp Hrs and if not the weak suck off the strong and thats a very very wicked thing........
Okay got it????????????
In series VOLTAGE adds,,,,,,,,,but NOT CCA or Amp Hrs
If two same voltage in paralell, voltage stays same, but CCA and Amp Hrs is additive