As stated above. The circuit on the stanchion is 30amps. Each circuit on your boat is also 30 amps. I have used a splitter without issues, however you'll need to balance the load manually.
A neighbor fried his cord this summer when they were trying to run the A/C, battery charger, hotwater heater, TV, the stove and then tried to run either a coffee maker or microwave. In his case, the breaker on the boat did trip, but not the breaker on the stanchion and not before he fried his shorecord.
Suggestions to split a 50amp to 2 x 30amp good, but only if you have a 50 amp circuit available on your power stanchion (which is typically only seen on larger slips).