Originally Posted by 100bikes

Agreed that derailleur spring determines the float of the derailleur with both pulleys in the same vertical plane as the (largest, in this case) cog.
I still contend that the dropout alignment matters. With the large gear range tandems sometimes have, this would be an important factor.

The small chainring and large rear cog combination is in the middle of the range of tensions that rear derailleur needs to
handle(small front chainring/ small rear cog being the the most difficult for the derailleur to handle - carrying a lot of the
chain wrap/weight as well). The shifting may not be affected through the ability of jockey pulleys to float around a bit.

The photo shows that there is a clearly a misalignment. A tandem with a forged dropout such as this should/would definitely
have the hanger in alignment to accept the B screw to hit dead center from the factory/builder.


Check the most outward edge of the derailleur for any indication of damage;
and
Check the hanger for paint buckle

From the posted photo, it is difficult to determine any /all issues.

Two photos that would help the group diagnose the issue:
One from behind the bicycle , showing the rear derailleur with the chain on largest cog in rear and smallest ring in front, to show alignment;
Photo showing the face of the derailleur from the side having stated that it is all original, except the cassette.