Which shouldn't be difficult for two operators under common ownership.
They might have the same ownership, but the two TOCs have different types of contract to run trains - one is DfT NRC and the other Open Access via ORR/NR.
For one thing, XC revenue goes to the DfT/Treasury, while GC (OA) revenue stays with Arriva.
In the past, the DfT, and the regulator, have been very unhappy with franchise/open access mixtures.
Even within the DfT/franchise setup Virgin had to unpick some of its WC and XC joint management (when it ran both franchises).