According to "Wanted in Rome" and the Parco Archeological Del Colosseum website, a 2nd on-site ticket office will open on 10/18 at Largo della Salara Vecchia in addtion to the one at Piazza del Colosseo. The number of tickets available for purchase at both on-site offices will also be doubled. Additionally, ALL tickets, not just those for the underground and arena, will be "named" in an effort to discourage bulk purchasing by bots, scalpers, etc. You will provide ID info at purchase, and ID will be checked at the entrance.
https://www.wantedinrome.com/news/italy-rome-colosseum-new-ticket-system.html
https://colosseo.it/en/opening-times-and-tickets/
The WIR article says that tickets are available for purchase at www.colosseo.it website but that's not exactly correct: clicking "Buy ticket" just transfers you to the Coopculture ticketing site. Both sites are "official" though; the Parco website is a bit more informational than the ticketing site
The Coopculture ticketing site is also active 24/7 whereas the Parco site is only "up" during hours the park is open...or so has been the case for me. The question I can't answer is whether tickets that include the underground will be available at the 2 on-site offices. To my knowledge, that hasn't been the case so far but hopefully someone with firsthand experience can give us the yea or nay. :O)