I have a little problem.

I have a form that needs some input. At some point, a user can press a button which shows another form modal.
That form on turn, has a button that needs to show another form. That form however, is a MDIChild, and is also used elsewhere in the program.
This results in me not beeing able to show that form modal, nor can I show the second form non-modal since the first is already modal.
What I'm looking for is a solution that doesn't require me to redesing half the application, so I'm looking for a way to wait for a form until it unloads. Currently I'm looping through the forms collection, to see if it's still there, but this uses quite some resources. I can't implement Sleep, because that will freeze up the other form.
I need something like WaitForSingleObject, something that will wait without using much resources. It tried WFSO, using the hWnd of my form, but that doesn't quite seem to work.

Anyone have any suggestions?