FYI:
Circular breathing is not inhaling and exhaling at the same time since that is physically impossible for humans to do.
Anyone who says they can is full of it. You cannot have one set of muscles relaxing and contracting at the same time.
Circular breathing is a practiced technique of using your mouth to push out air that you are storing in your cheeks while you inhale through your nose. When we are low on air while playing we push some into our mouth to use while we take a quick deep breath with our nose then switch back to using air from our lungs. Anyone can do it with practice.
Here are a few simple excersises I used to learn: ( I play didgeridoo)
http://www.woodwind.org/clarinet/Stu...Breathing.html
http://www.didgemusic.com/instruct/circ.html

Those sites explain the basic concept and will show you some real simple ways to learn how its done. With smoke its easy as pie but to do it while sustaining a note takes practice.