The full list of participating operators is avaliable here: https://www.gov.uk/guidance/2-bus-fare-c...the-scheme

For the North East (and surrounding areas), the following are taking part:

Abbotts of Leeming
Arriva Durham County
Arriva Northumbria
Connexionsbuses
Cumbria Classic Coaches
East Yorkshire
Go North East
Reliance Motor Services
Stagecoach North East
Transdev (all North West & Yorkshire divisions)
Weardale Motor Services