Use the second parameter to FindWindowEx() called "hwndChildAfter":
A handle to a child window. The search begins with the next child window in the Z order. The child window must be a direct child window of hwndParent, not just a descendant window.
So you should be able to pass the first textbox handle in to get the next one like this:
txt2 = FindWindowEx(hwnd, txt, "Edit", vbNullString)