The 27M is listed as a deep cycle battery and not a dual purpose one although it can be used for starting the small diesel. The PO probably chose the combination as not to burden the deep cycle battery with cranking loads and keep the house bank and starting bank independent of each other. My question would be, "how are they connected?". If connected by a 1,2, all,off battery switch I would suggest that for starting and motoring only to use the 24M and when under sail with the engine Off to use the 27M. I would refrain from combining them (both or all), as they are of different size and composition, but only for an emergency. This is not a critical warning that would cause instant harm but more of procedural recommendation to use the batteries as they were intended. If both batteries are good and they fill your needs then wait until they need replacement and get a pair of similar Group 27, deep cycle batteries. They can start your engine and could be combined to provide a house bank of double the size. It would surely cut down on battery switching as you could leave the dock and return on the same battery(s). It would be very appropriate to carry a fully charged battery pack to serve as a backup for starting the engine should the combined house batteries be drawn down. Also remember that the engine's alternator charge is only directed to the battery selected at the switch and only when combined at "All" would both batteries receive charge.
