With an in-series config, normally vac 2 fails more than vac 1, because it's running hotter due to vac 1's exhaust being fed straight in to it. Suppose you could can swap them round after a while to even out the wear but I've never bothered. I used to get years out of my Ninja's motors just by not abusing them! Just scrapped one out of my current machine which was date stamped 2003!
John if you're moderately capable with a screwdriver in your hand you can change vac motors in the Ninja no problem, and service the pump too. Make sure you've got a couple of the foam gaskets to seal the motor on to the machine manifold. You can often re-use them if they're in good nick but sometimes they tear trying to get them off the old motor.
The only other specialist thing you might need is a crimping tool and some
5mm bullet crimps. I can't rmember off the top of my head which way round they are... I think the male connector is on the motor wire and the female is on the wiring loom, so obviously you'll only need one or the other.
When changing vac 2 you need to re-fit the exhaust hose. I always drill 3 holes on the motor's exhaust horn, put loads of silicone on the hose to seal it in place, pierce the hose with a bradawl and then put a long pop rivet through the holes to fix it all in nice and tight. You don't want exhaust leaking out into the machine just in case it gets a bit damp in there!
Some vac motors come with a flange on the inlet side, which you need to remove with a hacksaw to fit on the machine. When sawing, try to have the motor turned over so gravity lets the metal swarf fall out of the motor rather than in to it. After sawing, hold the motor so the hole is facing the floor and tap the motor gently whilst giving it a good old clean out with a vacuum. Sounds complicated but it isn't at all!