Hello @moritzh76280848,
Recommendations only supports a single key per request for item based algo types (e.g., viewed this, viewed that; viewed this, bought that; bought this, bought that). However, it has a keyless type that is meant to be based on a visitor's set of behaviors (i.e., user-based recommendations).
You could in theory make multiple requests (each with a single key) to Target to achieve a multi-key recommendation scenario and then use some client-side JS to merge the results.
Hope that helps.