We created solution like you need some time ago.. It's not possible out of the box, but you really could add a few 'inline' calendars to custom popup and synchronize using JS API.. it worked smoothly at the project I saw.
Do you have the JS code? If you can provide it will be of great help. And also I wanted to achieve it using Richfaces but there is no attribute in <rich:calendar> which can help us in achieving the requirement. We can achieve it using Jquery but there is lots of prob in compatibility with jquery and richfaces. You can go through this link to see that my requirement has been met in Jquery