maztec's dashboard

This would depend on what license the two users have.

You could set it up so when one user proposes to the second user to combine synths, it includes a license proposal - make sure the dialog shows the user what license the other person is using. Then, mark both users as the owner of the combined synth under that license - if the other user accepts. At that point you have shared ownership and responsibility.

You could provide individual ownership/responsibility options and allow the users to allocate risk by selecting essentially bartering for ownership in the offer.

However, I think the solution that makes the most sense to me is for each user to retain ownership in their photos, with their original, compatible [will have to check for that] licenses. The new, derivative work is then a collaborative project, with the entirety being a shared entity under both licenses, but contains an indemnity clause that if any challenges come regarding individual images - those go to the owner of that image, not the union of the images.

Or, well, yah, you could just give them a wikipedia model shared ownership and responsibility - and notify them that it defaults to that license once projects are combined.

As long as the user is made aware of the licensing change or adaption when they merge their projects, I do not think there would be a problem.

However, I think either user, independently, should be able to remove their images from the combined unit - and if the unit ever reduces to a single user, ditch it entirety - with a notice to the remaining user.

The collaborate feature should allow more than just two users to combine. It should be designed so that multiple users can be added.

For example, my Sagrada Familia synth is minimally synthy - but if I were to combine it with other Sagrada Familia synths [or borrow a bunch of CC Derivative works from Flickr], then its synthiness would increase. The more people that join in, the more valuable to everyone the final synth would become.