How really depends on exactly what you're trying to do. If you want to update dlg1 when dlg2 is dismissed (OnOK) then:

Create a CString member variable for edit1 in dlg1, say szText1
Create a CString member variable for edit2 in dlg2, say szText2

if (dlg2.DoModal() == IDOK)
{
szText1 = dlg2.szText2;
UpdateData();
}

If you want to update dlg1 as the text changes, i.e. reflect the keys that the user is typing, you'll need to pass a reference to dlg1 to dlg2 and update dlg1.edit1 in response to EN_CHANGE (I believe) notifications from edit 2