Oh, that makes sense 
But both tunnels are terminating on the same site (one is primary, the other is fallback). I think I've to create some kind of grouping like in the exmaple from rtilak and put the MIP on the group?
Sorry for those annoying question. But the partner company told us that this is one of the working devices for their tunnel config, but they don't give configuration support and I've never worked with Juniper devices before. So maybe there are more questions 
