This is a great collection of wisdom as I have a no name aluminum tandem frame from about 2000 and when the stoker chain got real loose I tried to
adjust the eccentric (sp) in the main crank area to tighten it and I could not move it. I just wound up buying a new unstretched chain to solve the poblem.
Maybe someday I will be able to break it loose with the info here. I fear dis-similar metal corrosion between the aluminum frame and the parts will act like glue stronger than a weld.