Hello Oliver
Load balancing in general across L3 links in such a scenario as the one you describe is typically achieved using equal cost multipath routing (ECMP) typically configured using routing protocols such as OSPF, EIGRP, or BGP in conjunction with CEF.
For multicast traffic, PIM is the most commonly used multicast routing protocol. However, PIM does not have its own load-balancing mechanism. Instead, it relies on the underlying unicast routing protocols and the router’s load-balancing strategy. So for your particular scenario, if you ensure that you have ECMP configured correctly for your underlying routing configuration, you should also see the appropriate load balancing for your multicast traffic as well.
Ultimately, whatever you end up doing, you should examine the results using a network monitoring system that will either verify that load balancing is successful, or will indicate that there is room for optimization. Let us know how you get along in your configuration.
I hope this has been helpful!
Laz