They do both become live for the network, however, for them to break you would most likely be losing layer 2 connectivity between the two ports. HSRP communicates using multicast and, without multicast routing enabled, these messages would only be broadcast into the layer 2 environment that the subnet is configured on. If this L2 environment was split then both would become active and you would have two instances of the same subnet on your network. Access out of each switch would still function properly, but return traffic would choose one or the other router as the prefered path so only half the network would remain functional.
A second possibility is that the L2 environment remains active but you begin unintentionally (or intentionally for testing) dropping the multicast traffic. This would result in both interfaces becoming active and responding to ARP requests for the VIP address. If this occurs you will see MAC Flapping on the switch or switches connected to the two routers because each router will announce itself as the correct MAC/interface for that IP.
