Take a photo with one device.
The device puts it in the Camera Roll.
It is also cited to PhotoStream.
You can then run the Photos app on the other device.
View PhotoStream in that other device and you'll see the photos there.
To save the photo on the second device share it from the PhotoStream to the Camera Roll on the second device.
In addition to Ralph's comprehensive summary you may find these short support pages useful:
- iCloud: Photo Stream FAQ
- iPhoto and Aperture: Using Photo Stream
- iCloud: Photo Stream Troubleshooting
- iCloud: Using and troubleshooting Shared Photo Streams