One of the best Hydraulic Disc Brakes for Tandems has been the Hope Mono M4's, Or M6 if you want the ultimate. One of the problems I found, even though I use the 200mm discs, has been the heat generated from the disc's. Never had a problem with efficiency when hot but "Alpine" riding may cause a problem.
Hope have bought out a new Brake and the new form of the M4 looks as though it will be better suited to Tandems. Haven't tried them but well worth a look.
Hope Web site
Review of the brakes is shown below
And a better look at the brakes is shown below