I do something very similar in formset all the time,

Here's all it goes:

On opening the forms set, only my main fomr is visible. all other one exist but are invisible.

All my secondary forms have windows type = 1-modal.

When I want to call one of the modal form, I just make it visible like this:

thisformset.mainform1.enabled = .f.
thisformset.DetailForm.show()


And when I close the Detailform I go like this:

thisformset.Mainform1.enabled = .t.
thisformset.DetailForm.hide()

It goes very well form me. The only draw back is that all the forms are open on load of the formset so it takes a little while but when your in the formset it goes very fast.