You can certainly install two single pole breakers in this box but I am not sure that you will satisfy the requirements of the ABYC standard.
When you plug in to shore power the hot is supposed to be hot and the neutral is supposed to be neutral, but if somebody wired the plug backwards and you only used a single pole breaker you are a bit out of luck. In this case even it the breaker is off the entire circuit would be live and anybody working on it might get fried, despite the breaker being off. That is why your primary breaker has to be a double pole one with a reverse polarity light or alarm. If the light is on or the alarm is alarming don't turn on the breaker