From Rohloff, here are the recommended chainring / rear sprocket combinations: "If mounted on a tandem or if the rider weighs over 100kg, the following sprocket ratios must not be undercut: 34/13, 38/15, 40/16, 42/17."

For your timing rings, anything between 34t and 42t will work. If you're building a custom tandem you could even build it to be chainless using the Gates Carbon Drive - drive belt and tandem sync belt, although using the Gates timing belt requires that you use either a 28.5" or 27.75" stoker compartment based on the available timing sprocket/pulley sizes.