The diagram at the left shows the different features of the pens.  Your projector comes with two pens (color coded blue and orange).  Both pens operate in the same manner, and the blue and orange colors are only there to differentiate between the two pens.  Two people can write on the whiteboard at the same time as long as one has the blue pen and the second person has the orange pen.  For example, if you try to use someone else's blue pen along with your own blue pen, they will interfere with each other.  Make sure the pens are blue and orange.