You need to have a form manager. But in absence of a full manager you can use simple properties:

ADDPROPERTY(_VFP,"FormOne",.NULL.)
ADDPROPERTY(_VFP,"FormTwo",.NULL.)
...

DO FORM TopLevelOne TO loForm
IF VARTYPE(loForm) = "O"
   _VFP.FormOne = loForm
ENDIF
...

DO FORM TopLevelTwo TO loForm
IF VARTYPE(loForm) = "O"
   _VFP.FormTwo = loForm
ENDIF

In the destroy event of the respective form set the reference property to null; i.e. for TopLevelTwo form add the following:

_VFP.FormTwo = .NULL.

Now you can reference either form via the appropriate _VFP variable. First always test for form instance:

IF !ISNULL(_VFP.FormOne) .AND. VARTYPE(_VFP.FormOne) = "O"
  *-*  Do something with the form
ENDIF