You will need 4 cables to the dish, then split each of those out to the 4 inputs to each SWM box. Each of the SWM8's handles only 8 tuners max, no matter the number of outputs, the SWM16 'dual' output it really just two SWM8's in one 'unit', the only advantage is that one doesn't have to 'split' the dish outputs into multiples, it still has the 8 tuners per output leg restriction, it simply has two SWM outputs.
One can get SWM splitters anywhere along the output lines. Putting the SWM power feeders at the SWM location (garage, where I guess there is AC power), rather than at the end of the 250' RG6 line, is the way to go on that.
And yes, the cost (high) of the SWM16 really means that simply going with two SWM8's with some dish splitters is the way to go (pretty cheap, easy to get, not that big of a deal). I actually have never actually had my hands on one, it's a bit of a rare thing still.
Actually, looking up the current costing, it isn't that big of a deal, about a 30-40% more for the 16 v. 2-8's, and the availability is high.
And yes, the SWM output can be 'split' more than once; there are 2/4/8 type splitters, you can put them wherever in the system is needed, they are cheap.
Tons of wiring diagrams out there on mr. internet, on both the sales sites and others. Lots of places have cheap 'jumper' coaxes as well, I used to buy those in bulk for putting together systems, as well as the splitters.
Tons of places to get all this stuff, Solid Signal and others.
I use http://www.techtoolsupply.com/
but that's simply because they were among the first to have all the little bits and pieces, and are a bit more 'industrial' in nature.