Explanation:
Sub1 : An ExpressRoute circuit connection authorization
Sub2 : An ExpressRoute circuit connection
You can share an ExpressRoute circuit across multiple subscriptions.
The circuit owner is the administrator/coadministrator of the subscription in which the ExpressRoute circuit is created. The circuit owner can authorize administrators/coadministrators of other subscriptions, referred to as circuit users, to use the dedicated circuit that they own.
Circuit users who are authorized to use the organization's ExpressRoute circuit can link the virtual network in their subscription to the ExpressRoute circuit after they're authorized.
https://learn.microsoft.com/en-us/azure/expressroute/expressroute-howto-linkvnet- classic#administration