Pretty much every tandem I have had since 2005 has had a long cage derailleur because they have all had triple cranksets. A long cage is a given with a triple up front. With an 11-42 in back and a double up front it's six of one, half a dozen of the other. First thing, besides the chain wrap, there is a clearance issue with a 42T cog. It is doubtful that a medium cage derailleur was designed expecting the top jockey pulley to duck under a 42T, but ANY rear derailleur built to clear a bigger than 40T rear cog will also have a long cage whether you need it or not. If the o.p. was running a 1x setup, an 11 - 42 might not tax the wrap capabilities of a mid-cage RD but, that 42T ... chances are good the OEM RD would not be able to cope, even with only 31T of total wrap. With a 2x plus 11 - 42 deffo new (long cage) RD time. That was easy.