iMore reported you CAN reference image files in Photos.
http://www.imore.com/what-you-need-know-about-photos-os-x
Whew...that's a big deal.
But you can't merge libraries.
Keep in mind that this is a pre-beta version. I'll reserve my criticism for when we actually know the feature set of 1.0. Allowing for multiple libraries certainly requires the ability to merge them or otherwise import libraries into one another. This sounds more like an unfinished feature, than one that is missing.