From listening to the announcement on Zoe Ball's Breakfast Show this morning, the reason Zoe gave for needing a 'co-host' is because she can't commit to doing Radio and ITT Mon-Fri, so Rylan will present the show solo some days. They may present as a duo on a Friday since that's a live and extended show? But I think one of them will do Mon and Tue (Tue are usually recorded on Mon), and the other Wed and Thurs (Thurs recorded on Wed).
Rylan will probably continue with This Morning, he only really does it once every few months - or regularly in the Summer.
Xtra Factor has been axed and hasn't aired since 2016.
I always feel using Youtubers as "celebrities" is scraping the barrel.