*** Nevermind! I figured it out. I needed to duplicate the cell.
I'm trying to link a door in my treehome[sgstreehome] to a door in a copy of the SideShipElvenInt, which is already used in Side's Sailing Ships.esp.
I loaded Side's Sailing Ships.esp (esm) and sgstreehome.esp into the construction set. First I located SideShipElvenInt in the cell view. Then I tried to make a copy of the interior by renaming the Editor ID to sgsSideShipElvenInt. Then I renamed the door in SideShipElvenInt to sgsSideShipARDoor01bInt. Then I linked that door to the door[sgsTrapDoor] in sgstreehome.esp.
I reset Side's Sailing Ships.esp flag and started Oblivion. I selected [sgsTrapDoor] and it took me to SideShipElvenInt, not sgsSideShipElvenInt. When I tried to return to sgstreehome, I was transported to the deck of the ship. So what I thought was a copy of the interior, really wasn't. I checked the door in sgsSideShipElvenInt and it points to sgstreehome[sgsTrapDoor]. Sgstreehome.esp loads after Side's Sailing Ships.esp.
So now it seems I need to place a copy of the SideShipElvenInt base object into sgstreehome.esp. Normally I would create a new reference to the base object and drag it into the render window. But since it's an interior, what cell do I add it to?