How to store/send linking events using event sourcing?

I have an endpoint which is linking entity1 to entity2 (many to many), so after they were linked I need to send an integration event. I am using event sourcing and outbox pattern. So what must be the aggregate of my event? entity1 or entity2 What must be the topic of my integration event? entity1 or entity2

Also in the event store(where I store domain events) I need to specify streamName, but as I understand it needs to be buisness valued entity name, so it cannot be like entity1_entity2 (the name of linking table)?