Will,
As mentioned above, I sometimes use two ninjas hooked up together in parallel.
I use two 6ft lengths of vac hose from each machine which connect into a Y junction that I had fabricated for me out of 1.5" aluminium tube (I'm lucky enough to have a cousin who works at a local aluminium fabrication plant) A single length of vac hose then runs off the front of the Y junction to the wand.
You can still only use one solution line from one machine, as it's impossible to hook up the two pumps together to work in tandem (I can expand on that if you want even more of a science lesson
)
This configuration (on paper at least) gives double the airflow of one machine, with the same vacuum lift. I use it for cleaning faster than normal with one machine, so it's very useful on big commercial jobs and even large domestics. The reason it's quicker is that you don't really need a proper forwards drying pass on most carpets, so you can chuck the wand forwards quite quickly and get on to your next wet pass without the worry of leaving the carpet too wet. On big domestics, I can use the setup to give me another advantage - carpets dry in ridiculously quick time!
If you connect two machines together in series, you'd be increasing the vacuum lift but the airflow would remain the same (again, only on paper, there are real world differences). This means you can use extended hose lengths, such as recently where I had a job on a 2nd floor flat with no lift. I really didn't fancy dragging a machine up all those stairs, plus there wasn't room to swing an anorexic cat once you got up there. So I sited 2 machines outside the front door, had a bit of piddling around to do with the electrics, and then dropped 100ft of hose down from the flat out of the landing window to the front machine.
I wouldn't ever use a twin vac machine at more than 50ft, the drop in vac performance at 75ft is shocking. I do also notice a drop in moisture removal when using the parallel setup at 75ft compared to 50ft, but it's still definitely useable. 2" hose from the Y junction to the wand would no doubt help here, but I only own 1.5" so have never tested that theory.
The bottom line is this. Pi**ing around hooking up 2 portables is a mug's game
I've only been playing with it because I'm a compulsive gadget man, I love messing around with machines and experimenting.
I've had other things to be spending my money on in the last couple of years, but now I've got some other purchases done and recouping money, my next capital expenditure is going on a truckmount setup. Seriously, it's the only way to go. Even a small one is better than plugging all those wires in, even if it doesn't give you much advantage in the vacuum department.