You can only have 1 client file open at a time in the same year's program.

Yes, if you mean to look at Client A for 2018 on one Screen and Client A (or B) for 2017 on another screen. As @Just-Lisa-Now-  says, you can not look at more than one client in a single year. 

If you do not have two monitors, you can still have two year windows open and view a client in each, but they may be a bit tiny. Alternatively use Alt + Tab to switch between screens.