I did click on the api.stackexchange.com you posted and it didn't respond to order parameter. As the SEAPI has no ambition beyond translating Pythonic queries to StackExchange API, there is little I can do. Maybe the other app uses a different query or takes all favourites (I guess, rarely more than a few thousands) and performs the sorting? I do not know.

SEAPI is not under active development. But if you want some changes, why not forking the repository - I would be happy to accept some pull requests. :)

x = so.fetch("/tags/{tag}/related", tag=tag1), y = so.fetch("/tags/{tag}/related", tag=tag2) and the rest is Python (e.g. get tag names and make set intersection, if it is what you have in mind).