I'm going to answer two questions here.. First the rain question: Motosat domes are rated "Best" for rain fade, They are the last to die in a rain storm,, I suspect by about one minute or so but Motosat does not feel it is necessary to wax the dome, King Dome sells wax at a very high price for their domes. One of the reasons I like Motosat.
The T-2 Automotion I have does work very well when in transit, It's a DP model and is stacked (Meaning they put both outputs of a dual LNB on one cable.. I know it's done, but alas, the explanation of frequency domain multiplexing can be complex)
It does drop out when I go under a bridge or tunnel (of course) or in some other places where it's view of the southern sky is blocked (S.Western sky normally)
The DP (Dish pro) version is STACKED, and if you are going to run multiple receivers you need to have one that is STACKED
Motosat says they have had problems with these and thus they are not, currently, in production (I have no idea what the problems are, mine works great)
I have a dual receiver hooked up at it's base.
Hooking up 3 or 4 is possible, you need the proper adapter or adapters on the end of the indoor end of the cable is all. Basically a dual lnb is TWO lnb's in one case, They are often described as "H" or Horzontal and "V" or Vertical though in reality they are more likley R and L (right and Left) this refes to the nature of the signal they are looking for. (Letters describe polarization)
A single LNB has only one LNB and a switch on it's antenna to change from V to H or R to L
A Single lnb can feed a bunch of receivers, so long as they all want either one polarization or the other But if one set wants to watch a V transponder and the other wants an show that is on an H someone looses.
A DUAL lnb (Which the DP version or any other stacked version is) has two LNB's one dedicated to each side and a way of choosing between them (Either a switch, or pair of switches or in my case where the receiver looks for them)
I could add a 2nd dual receiver quite easily as both polarizations are live all the time so if Receiver one is watching a "V" and receiver 3 wants an "H" no problem. Each transponder is on a different frequency and the nature of a block converter is that it can pass half a dozen (or more) transponders all at the same time if the receivers want it to, it is just translating frequencies
My best suggesiton is
http://www.motosat.com and click on the "Contact Us" link