Davey Rimmer will play bass in Uriah Heep and in Judas Priest. Uriah Heep said so themselves, hours after Priest confirmed him as Ian Hill's replacement.
That answers the question everyone in both camps asked first. Rimmer has played bass in Uriah Heep for over a decade and had already learned the Priest set as cover during the European tour, so the band that loses him would have lost him at the worst possible moment.
Two jobs of this size work only if the calendars do. Priest tour heavily and have a new album recorded, Uriah Heep are a touring institution in their own right, and a bass player can be in one place at a time. Somebody will be working around somebody.
It is also the arrangement he already had in part. Rimmer plays in Elegant Weapons as well, the band with Richie Faulkner of Priest in it, which is how a Uriah Heep bassist came to know the Priest set in the first place.