Q. Can I have ExpressRoute connected to multiple on-premises locations?
A. When you purchase ExpressRoute you purchase a circuit via an ExpressRoute provider. Though the provider there are two connections to Azure routers to provide resiliency from line/equipment failure. The connectivity from your on-premises locations to the ER provider is between the customer and the provider who should be able to extend the connectivity to multiple locations, for example multiple on-premises locations.
Note that the type of ExpressRoute connection impacts the connectivity. If the connection is Layer 2 then the connections can be to one location, multiple locations or some other network such as MPLS or even another provider. If it is Layer 3 then it would integrate to an MPLS network.