The two true statements about IPv6 router advertisement messages are:
- They use ICMPv6 type 134.
- They are sourced from the configured IPv6 interface address.
The advertised prefix length in router advertisements can vary depending on the configuration and is not strictly limited to 48 or 64 bits (commonly 64 bits is used for hosts, but not a requirement for the advertisement itself). Therefore, the statements regarding the prefix length are not correct in the context provided.