While I think it would be ok to put another charger in there and hook it up to the other two batteries...it seems to me that for space and weight reasons, you should be better off with a 3 or 4 bank charger.

I'd also suggest that you copy and paste your question into either (or both) the main bass boat forum or the electrical/TM forum for a lot more views by guys who know more than I do about onboard chargers (mine is old, and as long as it works, it's not getting replaced).

Be aware that some battery types (conventional lead/acid) like 10 amps and some (gel cells) prefer 15 amps each. I'm a big fan of gel cells, because I can't check the rearward cells for water levels. My charger is a "Dual Pro". When it went TU, I called the factory and they offered me a killer deal on a 15 amp/bank, but I had to send the dead unit back for refurb. There are many newer products out there now.