If you hook them positive to positive and negative to negative, you will get the results your looking for but you have to realize a few things. First of all, you won't get that much of a charge out of your engine unless you run it a lot. You will be taking the chance of exhausting both batteries with your trolling motor thus possibly leaving you stranded without enough power to crank your engine. With the batteries connected this way, your voltage will stay the same but you will increase your amperage draw reserve which can be a good thing. In short, its best to leave them seperated. You can buy an adapter out of bass pro that will isolate and charge your other battery off of your cranking engine but again your gonna have to do a lot of running with your Mercury to see any benefit. Bass fisherman often do this because they run great distances between holes and are able to take advantage of it. Hopes this helps.
The only way to have a good fishing spot is to make it yourself!