Hi Mark, I've never ridden one, or even know anyone with one, so I'm not really sure what tandem friendly is....but I assume its quite heavy, so I'm thinking its not very good on hills, and is big and bulky so is a major headache if it needs lifting over fences and stuff, and its long wheel base is not so good in tight twisty stuff.
Don't worry there's no fences or off road of any kind. I think you'll be fine, the route is nearly all flat. There are 2 inclines that I recall, that I would'nt even class as hills, the rise up to Finsbury park and a short incline in Walthamstow. We do cross over a small footbridge over a canal, but its has those wheel guide/rail things so bikes can be pushed/rolled up and down and it has only about 10 shallow steps on each side so that will be fine. And we also cross a bridge over the A12 by Victoria Park, but its a ramp not steps so can be cycled, you may need to be dismount on the bit where it doubles back 180 degrees on itself just to be extra safe. I'd love to have a tandem along, they seem to guarantee extra smiles
and I'll happily wait or help with any pushing on those bits I mention above.