It would work, but it's not really optimal.
You could do what you are saying without using a "doubler". Connect the microfilter to the current socket downstairs and plug the phone into the phone part of it. Then plug the extension into the ADSL part of the microfilter and run it upstairs. The extension cable would have course need an RJ11 plug on it to be able to connect to the ADSL part of the microfilter. A filtered faceplate could also replace the microfilter if you didn't want something hanging out of your socket.