It is possible to run more than one instance of RIPng on a single router.
You can run a different process on different interfaces and you can also run multiple processes on the same interface.
One reason to do this may be that you want tighter control of updates between two interfaces. You can run separate processes on each interface and use route-maps to control redistribution between the two processes.
Another reason may be that you want to completely segregate routing traffic between two sets of interfaces. If interfaces 1 & 2 are in one process and interfaces 3 and 4 are in the other process. The router will exchange routing information between interfaces 1 and 2. It will also exchange routing information between interfaces 3 and 4. It will not exchange routing information from 1 and 2 with 3 and 4 or vice versa. This does not provide data plane segregation (as with VRF), just control of routing. If traffic arrives in on interface 1 and it is destined for interface 3 it will be routed out of that interface as there is single routing table.
The process names are only locally significant and do not need to match between routers. If you need to run multiple processes on the same interface you must change the UDP port number for one of the processes as there is no process identifier in the update. If you run both processes on the same port number the routing information will be merged on the receiving router.