You should not put more than one fan into a common duct.
Each fan should have its own duct run to atmosphere. If you try to combine them into a common duct it will bring with it a number of problems, including an unbalanced system with positive/negative air pressures which play havoc with the fan motors and could cause motor failure. It can also cause cross-contamination issues.
Also not to forget that incorrect installation of the fan could void the product warranty.
In residential and some commercial applications, an alternative to multiple fans into common ducts could be a Central Extract System Fan such as our Lo-Carbon Multivent MVDC-MS or Lo-Carbon Sentinel Multivent