Originally Posted by geoffs

I'm just about to go through the process of measuring our tandem for double chainrings too. I presume you're keeping thlhs for the timing chain.

Don't be tempted to use the Davinci titanium BB with the Davinci cranks as they use a non-standard bearing which isn't easy to source.
Unless you live somewhere very flat I would be looking at a larger cassette.
We're going to use an 11- 42 with a Wolftooth roadlink on a eTap WiFli rear derailleur. I'd look at using Velocity Cliffhanger or Blunt 35 if you have enough room. We run Schwalbe Supreme 2.0" tyres for touring on Blunt 35's 26" and they roll really well.