battery/perko question

is there anything wrong with using an optima blue top starting battery with an optima blue top deep cycle battery? We have them set up with a Perko switch.

If it is ok, how long should they hold a charge cold? went out to the boat after a month and they didn't have enough juice left to crank the motor.

Shouldn't matter if you turned them off. Sounds like you have a dead battery, or a short somewhere that is drawing them down.