It can be figured out by setting one VI as parent window and the others as child window.
LabVIEW provides the "Call Library Function" to call DLL. So you can come up with a DLL to set the window properties. The steps are:
1) Run VI's.
2) Get handles of the front panels by window names. (MFC function)
3) Set one of them as parent and others as child window. (MFC Function)