it’s possible (and done before), but not always trivial. For storing the events, you can simply define 2 event stores (each with a different storage), and configure each repository to use a different one.
However, that would mean that on the event consuming side, you would have to read from 2 sources.
Alternatively, you could adapt (probably extend or wrap) the MongoEventStorageEngine to base the collection to store data in on the aggregate type, which is part of the event to Store. Overriding the MongoEventStorageEngine’s appendEvents method should get you a long way.
Still, there is a challenge on the read side, to make sure you read from the correct (or all) collections.
Hope this provides some pointers.