1. Double-click on your door in Tamriel worldspace to bring its properties window up. Do NOT close this window.
2. Go to your interior cell you want to link that door to and bring it into view in the render window.
3. Back to the pop-up from #1 above, Click on the Tab called 'Teleport'. Put a check mark in the 'Teleport' box.
4. Click on the button called 'Select Reference in Render Window' and double-click on the door you want to link.
5. Click on the OK button to close the window and the doors are now linked properly.