Whisk itself (https://labs.google/fx/tools/whisk) was released a few months ago under the radar as a demo for Imagen 3 and it's actually fun to play with and surprisingly robust given its particular implementation.
It uses a prompt transmutation trick (convert the uploaded images into a textual description; can verify by viewing the description of the uploaded image) and the strength of Imagen 3's actually modern text encoder to be able to adhere to those long transmuted descriptions for Subject/Scene/Style.