This is an interesting topic, but as someone said, not for a non-marine-electrical mechanic to fool with.
Did, whoever started this topic, consider that there may not be a 3rd receptacle on the dock to utilize? IF there was a 15A household 3 prong receptacle additionally on the dock do you know that connecting to that wouldn't blow everyone's breakers on your dock? You'd become a very disliked neighbor very quickly.
Let's first get the terminology as I see it.
The requester has 2 -30A inlet receptacles on his boat. Probably Marinco twist lock versions. As an aside if they are the Marinco version, take some money and swap them out for Smart Plugs, both receptacle and cords. 1 is for the house, the other is for the AC. He fails to mention, that I read, how many and what kind of AC units he has.
My first question would be is he sure that the AC units, unless they are of the big box store variety, that they aren't reverse cycle units? Built in to the boat originally. I know ours is. IF they are then his question is answered.
I'll proceed presuming that they aren't.
IT then sounds like the wants to bring an additional 30 service into the boat, that he can use to run space heaters in his boat.
I would suspect that he understands the potential for creating a fire in his boat which of course has the potential to cause a dock fire which will impact not only him but others on the dock. VERY BAD outcome.
Using my own boat as my experience and since we don't know, as this often happens with people in general explaining things, there are huge gaps in the question, what boat he has and how it is configured, other than 2-30A inlets.
It is my supposition that the boat configured and built like it is could handle space heaters. What is necessary is some 'balancing' of his usage.
When connected to dock power it is presumed that there is 30A of power available and that may NOT be the case. Even for 1 inlet. A clamp meter would let you know how much power is available from each dock side outlet. Also, even if at a specific time there might be 30A available to his boat, IF another boat were to turn on a space heater (or other high amperage item) that may pull available power on that specific dock down as to the total amperage available. You might want to check with someone in the marina as to how the docks are powered.
IF you didn't want to go that route then the next best thing, and what you still may need to do anyway, if there is 30A available or whatever is available, is to turn everything off on the boat when connected to the dock, grab a pencil and paper, and start turning on items in a process of how you would be living on the boat.
IF say you wouldn't be using the MSR, then make sure ALL is off there. IF you want to keep it a little habitable, then turn on heat here but turn it down so the heater isn't cycling very often. IF your boat breakers or the dock side breaker doesn't trip as you start this process then turning more on, KEEPING track of what you are turning on.
The idea is to make your boat as livable as you normally would like it before a breaker is tripped.
At some point the 30A house breaker will trip and you can check your paper to see what you have on. Adjust from there. You can't have everything on the boat turned on at the same time. IF you think you should be able to you are being selfish and unreasonable. 30A is 30A is 30A period.
THIS IS A BOAT, not a house. The power into your house is a much higher amperage.
Once you have your paper you can mix items which are on.
You may find, as an example, that you can't flush the head, while you are watching TV, based on the other items you have turned on. IF SO, turn something else off while you are watching TV, just incase you might need to use the head.
We balance the use of the toaster with the use of the microwave and electric skillet. Usually at Sunday breakfast time.
Handling your boat electrical wants & needs conservatively keeps your boat safe and you a hospitable neighbor.
Forget any additional wiring, you are just asking for headaches as to keeping everything in order and NOT starting a fire because you go out and forget to make the necessary modifications before you leave.
Good luck and be smart.