You can include a mode_tile statement within the action_tile statement of the first popup_list.
However, a popup_list will always have a selection (even before the user has chosen something an item will be shown), so the behaviour you are looking to achieve may not be intuitive for the user.
I have one more issue. I want to have different contents in two popup_lists "mylist20" and "mylist21" depending of chooise "sp" or "tp". I have wrote a code see below but it doesnt work. Can you help me?.