How do i Keep final cut pro on one screen instead of splitting apart

By default, FCP will use one screen only.

To change that, you go to Window->Show in Secondary Display->...

The current choice will have a checkmark on it.

To change it Back to one screen, just use the same menu again.

Directly under "Show in Secondary Display" referenced above is "Workspaces." Click on that and a submenu opens that lets you save your current screen layout, give it a name and toggle among any of the custom screen layouts you have created. I find creating/saving a screen layout set up specifically for the tasks I want to accomplish at that moment quite useful.

Splitting apart? Are you using two monitors?

So you want the second monitor to mirror the first? Or you want nothing on the second monitor? Or something else?