Exactly! What's Joel's availability like, does he just do evening performances of Hamilton (7 days a week?), are there daily matinees as well? Carrie seems to have flexibility in her schedule at the moment. They keep going off on holiday every 5 minutes as well.
They live together, you couldn't make it any easier for them to rehearse.
They should've done a free gig or £10/ticket at most if they were testing something out. Carrie (and Joel by extension) can well afford the cost of small venue hire for an hour or two. It's basically promotion for Carrie's tour/a future tour of Montague duets