This is where it gets a little messy. From your EAT IN terminal, and in your EAT IN menu, you should be able to see the open ticket from the TAKEAWAY department. Select it and assign a table.
You technically can't push a ticket to a department, but you can take one from another department. The general rule is, be on a terminal in the destination department and in the destination department menu (ie EAT IN Department on an EAT IN terminal) that you need to open ticket to go too.
Hope this helps. I only have one terminal running at the moment, so testing was only brief.