Well, I can answer myself partially:
Write some code to open the supporting document over the first one. Don't
forget to first hide the userform. Put all of this behind a command button.
This works ok unless I press the new button rightaway.

The userform is called in an autonew macro (choose and put an adress from
the other doc in the main doc). Pressing the command button while still in
the Autonew
macro is not good because the main doc was not open yet. So after hiding the
userform both docs open, mixing text from doc 1 into 2. Great :-/

For now I removed the command button from the userform en put the code in a
macro that can be called from a toolbar in the main document. There is
another button to call the userform again so users can still switch easily.

Thanks anyway,
Flip