The website clearly states in several places that the double evening ticket is available March 10 through Nov 3 so April is not a problem. It also states that tickets are currently available through some time in June. I tried logging into my viva ticket account and kept ending up at the same point acraven describes which only shows Monday daytime tickets -- so something is off. In case this eventually works, here is the page that has a button to "book" the evening double shift https://www.giottosottolestelle.it/en/evening-tours/ but that led me back to where I started.
I did the evening double shift in fall 2024. It was amazing. I had about 5 minutes totally by myself in the chapel between the two shifts. I remember some trickyness in buying the tickets on the website but eventually I got it.. You do have to register first, I believe, for an account on Viva ticket. I think you end up purchasing it as 2 tickets, for example my receipt showed one at 19:00 hours and one at 19:20. I was not able to figure out how to combine the evening ticket with the Civic Museum the way you can with the daytie tickets but that was not important to me. Good luck!