Hi, Barry - sorry for the delayed response but your inquiry forced me into more research than normal. Here's a very "NON CERTIFIED ELECTRICIAN" response to your question.
There are two lines on a 50 AMP Service as you mention. If you were to count up all the various circuits that go into a 50 AMP service you would have over 100 AMPS easily with several 20 AMP breakers and several 15 AMP breakers dispersed equally in the system. Obviously, we are all familiar with the typical power management requirements in an RV and this is the way they are all built (whether 50 AMP of 30 AMP) so this isn't a surprise.
Where it gets tricky is how to allocate all of the circuits given there are two "legs" to the 50 AMP. You might have both A/Cs on one leg, but you're also going to have a lot of other huge power users on the other leg. For instance, Microwaves and Water Heaters both use a lot of AMPs. To a large extent, we try to balance out the two "legs" with an educated guess on how our customers would prefer to use their RV and everyone has different needs, so that can be difficult to guess correctly.
If, because of the way you use your Sanibel, you wanted to have a certified electrician switch one of your A/C circuits to the other "leg" and move something (water heater maybe?) back in it's place, that is relatively simple to do.
Your question generated a lot of discussion at our place in trying to determine the preferred method of allocation but I'm not sure we came up with a sure-fire answer that every customer will like.
Hope this helps.