transporting variable via post to another site

Hi there, I do have a multisite form. There are several fields on page 1 and several on page 2. Everything works fine exept of error handling. Which means if a user wants to go back from step 2 to one and has already filled in some data in site 2 he will loose this data for sure. It is not possible to transport the data via get anymor because the text is way to long. So how could this be done with post? I tryed to include a hidden text field, but since this must be another form (action links to page 1) I can't get the value of the entered data. Has anybody a good idea, or even experiance in this? Thanx, Andy