The specification of Neighbor Discovery mechanism in RFC 4861 does not exactly specify what happens if you have multiple routers on a network. In Section 6.3.6, it only says that routers that are known to be reachable or probably reachable shall be preferred to routers whose reachability is unknown or suspect. However, it does not say what should happen if multiple routers are equally reachable.
RFC 4191 introduced an indication of router preference into Router Advertisement messages - in the Flags field of an RA message, two bits are reserved to express the preference of a router which can be either High, Medium, or Low. You can actually configure this preference on a Cisco router using the ipv6 nd router-preference interface-level command. With this indication present, a host should choose the router with the highest preference as its default gateway. Again, however, if there are multiple routers with the same preference, the behavior is not precisely specified.
Finally, RFC 4311 tries to clarify issues of using multiple routers as default gateways, taking both previous RFCs into account. However, it only suggests that hosts implement some way to employ load sharing between multiple routers of the same preference, but again, no exact behavior is specified or required.
So the bottom line is - if there are multiple routers in a network, the actual behavior will depend on the particular operating system running on a host. Some hosts may choose only one router as their default gateway, some others may be using all of them in a certain way, but the exact behavior is unspecified.