You mean your partner is new to tandeming, and you want to ride a 24 hours time trial this summer? I wish you good luck!
Anyway, you are a lucky man and I'm jealous! When I was prospecting for a new custom tandem frame a few months ago, I asked Argos,
and they told me they don't make tandems anymore.
I'm sure many of us are awaiting to see photos of your tandem!