Thanks. It went better than I had expected, but after a while I stopped analyzing it and just rode the bike. Things just sorted themselves out.
We had some funny moments when she kept trying to coast and I was still pedaling away. She asked why I wasn't coasting on that section and I asked her why she wasn't pedaling! We also got to chat about spinning vs. mashing and had to compromise somewhere in the middle.
I'm interested to see if the tandem riding changes how she rides her single. She has the athletic ability to be faster, but somehow it's not coming together on the single even though she is a daily 5km bike commuter for the last 6 months.