Screen sharing on Skype is one-way only. Only one person in the call can share their screen with the other callers at one time. With the free accounts the sharing is limited to one-to-one. With the premium accounts you can do a one-to-many broadcast and share your screen out to multiple people at the same time.
This does make some sense, two-way sharing has to overcome a huge problem of mirror-in-mirror replication. The same sort of effect you get when you place two mirrors, facing each other, and look into them. There's an infinitely repeating, but gradually reducing, view of the other mirror, in the other mirror, in the other mirror, in the other mirror...
A similar thing would occur if you were to do a 2-way share.
If you use VNC for screen sharing you can actually force the two-way share to happen and see the effect for yourself.