It depends on the boat, where you paddle it and how you paddle it. I have two tandems, both with rudders that are not really needed. The first tandem is a 12.5' Hobie Mirage boat that we use as a tender to our sailboat. Of course, the rudder is needed when using the Mirage drives but not at all when paddling. That short boat tracks very loosely so it is easy for an experienced paddler in the back to steer while paddling.

Our second tandem is a Shearwater Double with a Smart Track rudder. I installed the rudder because I was concerned about maneuvering the 18.5' boat in the small meandering streams that we frequent in central FL. It turns out that the rudder is not really needed because the boat tracks nicely but is surprisingly responsive to steering strokes. The rudder is nice out in open waters but it actually makes the boat harder to turn in tight spaces at slow speeds.

In general, I suspect that a rudder will be more useful for very long boats that are hard to turn and for those that do a lot of paddling in open waters that are subject to crosswinds. Honestly, I would suggest that you paddle any new boat a time or two before making a rudder decision.

A more important consideration for paddling with somebody inexperienced is separation between the two cockpits. Inexperienced paddlers don't hold cadence very well so you end up hitting paddles a lot with closely spaced cockpits. Some boats, like the Shearwater Double, have the cockpits far enough apart that the paddlers can be out of sync with no fear of hitting paddles.