There are others that are probably better at this than me, but since no one has answered you I will try and help.
I would say its not a good idea. If the set up worked with out damaging the 12 volt, best case is you will draw down on one battery more aggressively than the other which will lead to battery life for both batteries because to use the 24 volt system you need to run them in series instead of parallel like you most likely have them set up. My major concern would be that since the two batteries are hooked up in series, even though you are going direct to one battery for the 12 volt, you are still drawing 24 volts. Maybe there is some sort of a switch you could use to pick the current you want...to me that incovieniece would make having the 2nd motor less useful.
I would try and find a way to add a third battery, even a used one to get you by for a bit or pull the battery out of the tow vehicle. I don't use my rear 12 volt very often (if you were not running other items off it battery life should not be the issue)but each time I do I'm glad I have it for things like snags, something falling over board, or time to head in and nice to have all the thrust at my disposal.
Hope that helps.