Hi Steffen,
How are you hiding the subform? If you remove the subform using the instance manager's "removeInstance" method, I believe you clobber the subform and its data. If you want to hide the subform but preserve its data, try setting the subform's presence to "hidden". To show it again, set the presence to "visible".
I have worked on projects where the data were "persisted" in form variables so that they could be brought back and remerged the way you are suggesting. So, that's possible too, but quite a bit trickier to develop.
Jared Langdon
J. Langdon Consulting, Inc.