Maybe someone can help me....

I have a 2 bank charger... one lead goes to my starting battery... the other to one of my two trolling motror batteries. Here is the question... can i hook one lead of my charger to my both my trolling motor batteries if i run them in series or parallel (its a12v system and im not sure if i should run them in series or parallel) and have them both charged? Its been perplexing me since I got the boat.... any help would be appreciated.

Keep on chuckin'