As far as I know, you can't write text in one field and just 'continue' writing in another field.
Therefore here are two suggestions:
1. In the 'Properties', 'Options' tab of the two text fields, check 'Multi-line' and 'Do Not Scroll'. Make the tab order of these fields consecutive. When you end writing the first, a Tab will take you to the second (even if not on the same page) and you can continue writing.
2. Have only one field, but make it Multi-line and scrollable. When you enter it after it has been written to, you can go down by the down arrow key to see all the text.
Hope this helps.