How big is the battery? Unless it is a bit weedy I don't think it should have a problem spinning over what is only I think around a 2.6L engine.
Anything decent that will provide say 600 amps plus of cranking power shouldn't have a problem at all - that will start a diesel of that displacement never mind a petrol. What CCA and Ah is printed on the battery? If its 70Ah and 600 amps CCA or more then I think you need to look at other causes e.g. faulty connection, bad earth, dodgy starter motor etc.
Unless you have twin engines and separate electrical systems or some sort of split charge arrangement I can't see any real benefit in having two batteries rather than one big one so one big one would be my suggestion. I have heard (on vehicle installations) that two batteries permanently connected + to + and - to - can self discharge due to current cycling between the two when everything is switched off, but I don't know if it is even common or even true - I've not seen it.
Just my 2p, I'm no expert on boat systems but I do spend a fair amount of time peering at vehicle electrics which is much the same as a boat but without the complication of salt water