You got 30 AMPS and 2 a/c units. Very strange...but you have a 1999 unit, so who knows what they were thinking back then? That means you have a 30AMP plug, so plugging into shore power, you can only use 1 a/c unit anyway.
It takes (in most cases) 50 AMPS to run both a/c units. 30 AMPS can run one unit at a time. As mentioned in another post above, there is a ($$) gadget you can buy that will autommatically switch from one a/c to the other.
My 2001 is 50 AMP....and has a 5.5 Generac Generator. This thing can run EVERYTHING in my motorhome at the same time. Both a/c, both TVs, microwave, lights, fans, the whole thing.
What I found over the years is that 98% of the time, ONE a/c unit is all I need. So...if you have two...alternate them yourself or buy that gadget mentioned. IMO, if the wiring ain't broke, don't fix it.