Photo Streams from two or more iCloud accounts into same iPhoto library?
Currently it can't. Photo Stream settings are detected via the iCloud preference pane and is global to your machine. Until it can support multiple iCloud accounts, iPhoto can only connect to one Photo Stream at a time. Since it's only a temporary store of the last photos taken I am assuming Apple will most likely not make this possible.

I normally move my photos from the Photo Stream to Flickr to achieve this since you can have multiple Flickr accounts.

Look into Dropbox with photo upload if you want to merge photos from multiple devices into one collective stream. From there you can use the Mac client and Folder Actions to automate importing, processing and other tasks as needed.