valet and two motor systems
I agree with Greg, do some diagnosis before spending the money on a whole new unit. First, leave the pipes out of the picture, plug in the unit, and use the power switch, or touch a screwdriver to the wire contacts, it's low voltage, should not cause a problem. When the motor turns on, check the intake and the exhaust on the unit. Do you have good suction on the intake and lots of air flowing out of the exhaust with a decent amount of force? If so, the unit is fine and the issue does involve the pipes. If the unit does not have good suction and airflow by itself, it could be a seal some place or the motor, does the motor still sound ok?
I think the vacuflo units that use bags are called maxum. I had thought about getting a maxum 7 which is the most powerful 120 volt unit you can get in that series, but the specs are not that good for the price, you can get units that are more powerful, motor does have a nice sound though. The two motor set up is interesting. If you have two motors in series, the suction, or water lift increases, but yes, this can cause premature motor failure, particularly failure of the second motor because the second motor is sucking in the heated air from the first motor. On the other hand, two motors in parallel increase the airflow, the issue here is that you're still only going to move as much air as what can fit through the hose and pipes, so it's hard to say whether there is any benefit to this set up. For a large building, it might make more sense to install two separate units in different parts of the building and have two separate installations. I have read a few debates about what is more important, airflow or suction, but the simple answer is, to clean well, you need a decent amount of both.
I think the Valet units are made by Lindsay Manufacturing, they also make the vacumaid and imperium brands. I have two vacumaid units, they are made very well and should last a long time.
Mike