Within Project1, execute the command:

SET PATH TO "..\aControls\,..\cControls"

Fox will then look for a File in Project1, if it cannot find it, it will look for it in aControls, if it cannot find it here as well it will look for it in cControls.

Notes:
(1) I assume your working folder is Project1.
(2) aControls can be a sub-class of cControls if necessary