Larry,
We have several water and stock pump systems in the mill like this. Keep in mind, water and paper stock (basically water) are non compressible fluids, so it works differently. The flow nearly doubles, depending on pipe size. If the pipe size is too small, then friction takes over and the flow is less. The discharge pressure goes up about 15% if the pipe size is right. Otherwise, the pumps run hotter than normal because the flow is cut back through each pump.
Keep in mind, we are interested in the discharge on the pump systems.
In the vacuum system, you are concerned about the "suction" side. Since the air is compressible, you may have a lot less friction effect.
To be a balance system, both blowers have to discharge out side or through similar bag systems or filter systems. Other wise, the path of least resistance will take over and one blower will do most of the work. You have to have the same resistance on the discharge side to get the same flow through each blower. The piping from the main ductwork to the fan should be as closely matched as possible also.