Quote Originally Posted by alpaca
I don't think it would be possible without changing the game code. Unfortunately something like tht can't be properly scripted because the create_building command doesn't upgrade existing buildings so it'd work only for the first building of each tree (well if you don't want building upgrades you could probably do something like that)
If that was the case, then the OP could certain unwind the tech tree so each building is standalone but has a certain other build as a pre-req (as seen in greater market for alchemy school)
Then when one building is finished, it will use create_building and destroy the other building after the creation. Reasoning behind creation first and destruction next is otherwise unit production facilities would reset to 0.

Wonder if that is actually possible!