... both rooftop A/C running cold! That lasted for a couple of hours and then - nothing. It all shut down..
... Once we got parked, I tested the solenoid and it was open. It had a good 24vdc signal, but no continuity. I have to assume it is blown.
First off, let me start by saying that I run two air conditioners, plus the rest of the normal bus loads, all the time using my 270-amp alternator ans a single, 200-amp solenoid. The solenoid has lasted eight years and 140,000 miles and is still going strong, so there is nothing conceptually wrong here.
You may just have a bad solenoid. But let me put the load issue to rest: There is no way the actual load passing through the load contacts can cause the coil to blow. Usually overloading the contacts has the opposite effect: welding the contacts in the closed position.
If you switch the solenoid on and off repeatedly under load, the arcing that inevitably results can pit the contacts and/or char them, leading to high resistance and which, left unchecked, can ultimately lead to an "open" reading. For this reason I recommend that any high-current loads such as air conditioners be switched off while the engine is being started and the contactor closes, and likewise before stopping the engine when the contactor opens. Note that I am too lazy to follow this recommendation myself.
But your description here suggests you have not had this system in place nearly long enough for carbon deposits to build up on the contacts. Moreover, if this was the problem, you should still hear the plunger moving when operating voltage is applied.
I suspect instead that the operating coil is damaged. This can come from improper voltage -- for example, using a 12-volt coil on 24-volts. It can also come from high back-emf when the operating voltage is suddenly removed.
The first thing I would do here is to disconnect the operating coil and measure its resistance with a quality ohmeter. The manufacturer of your solenoid should publish the specification for coil resistance. If the coil reads either shorted or open, it's a problem.
If the operating coil reads OK, it might be possible to open the unit to look at the load contacts. What make and model is your solenoid? Also, see if you can find a part number on it -- it's possible they sent you a 12-v model in error.
You can always run two solenoids in parallel so that each shares 1/2 the amp load.
Parallel 200 amp units will give you 400 amp capacity, 33% more than the one in the link.
Actually, this is incorrect. First of all, you can't simply double the rating when ganging relays in parallel. That's because there is no way to guarantee that half of the current will go through each one, and, in practice, one always gets more than the other. For double-pole relays, manufacturers usually publish the de-rating factor that needs to be applied when ganging poles together to increase capacity; when no specification is published, the accepted standard is 25%. So a pair of 200-amp poles ganged together is good for no more than 300 amps -- not 400 amps as your post would suggest.
But when it comes to separate, power-to-hold, normally-open solenoids, there is an even bigger problem: a failure in one solenoid would shunt the full current onto the other solenoid. For this reason, simple, single-contact solenoids are not permitted to be ganged together to increase capacity in this way. Certain types of contactors can be ganged, but it requires them to have "auxiliary contacts." The control power is wired through these contacts so that if any contactor drops (opens), then all contactors will open simultaneously. Operating this type of ganged arrangement requires momentary, rather than continuous, control signals.
For applications in the under-1000-amp range, it's cheaper to just buy a contactor rated for what you need rather than buying multiple contactors with auxiliary contacts and ganging them together.
All of that said, 200 amps continuous-duty rating should be sufficient for this application. Remember that the amount of current drawn by the air conditioners is not the controlling factor, but rather the amount of current that can be supplied by the alternator minus whatever is being used on the chassis side. We have a shunt and a high-quality ammeter on our inter-tie, and while inrush current can spike to just above 200 amps when the house bank (920 ah) is depleted, I have never seen it remain above 200 for more than 2-3 seconds. The alternator simply can not supply any more than that (the inrush spike is higher due to the chassis batteries).