Single valve has only one flow controlling checkvalve as only the intake manifold vacuum is used as the OEM system does. The can just installs inline between the crankcase evac barb and the IM vac barb so all emmissions remain intact and it just separates the oil mist/vapor from the rest of the suspended combustion byproducts.
The dual valve is used when you are turbo or centri supercharged as the IM becomes pressurized the primary valve will sense this and close preventing the boost from pressurizing the crankcase and then the secondary valve will open and use the inlet side suction of the head unit to continue the evacuation and then when you fall out of boost the valves return to the NA mode.
The dual valve can be used with a Maggie/ZR1 style top mount as well for an added evac source when driven hard, but is not neccesary for most top mounts. It would connect slightly differently.
For big cube builds or any with boost much over 10-12# the Monster can is the standard dual valve on steroids with over twice the capacity to deal with the added blow-by these motors experiance.
If a turbo or centri build did not have the correct checkvalving every time you would go into boost the crankcase would also get pressurized. Most FI kits come with a checkvalve to put inline to prevent this, but then when in boost the crankcase pressure will follow the path of least resistance, which is back flowing through the cleanside and bring the oil laden vapors through the main air filter/intake tube and that presents another issue.
Ask more questions.....especially any pertaining to a specific application or how oil ingestion affects the motor you have. Fleet owners of light trucks and vans buy these just for the fuel savings which avg's 1-3 MPG with the elimination of the detonation that puts these motors into the low octaine timing tables.
