Teleport Tunnels

I’m looking at the enclave sorcerers in the Book of Wishes and I have a question for you active game masters out there: Can the principal spell caster pass through the teleport tunnel or will his movement into the tunnel cause it to collapse? Here’s what it says in the book:
Teleport tunnel is a very useful tool for the enclave sorcerers. It creates a tunnel (the size of the area of effect) with one end at the principal caster and the other end at the auxiliary caster. Only two spell casters can be involved in the casting of this spell. The principal caster is the one who initiates the spell. This caster begins casting towards the other. After one turn of casting, the other sorcerer becomes aware of the teleport tunnel and has to open his or her end. The principal caster must use four power levels, while the auxiliary caster only needs to supply three. If the auxiliary does not respond, the principal will begin taking sustaining fatigue, even though the spell has not yet been cast.
Once the tunnel has been successfully opened, anyone or anything can pass through the tunnel and instantly be transported from one end to the other. Through these means, large amounts of people or products can be transported. This tunnel works much like the other teleport spells except that it is maintained. If either of the casters drops the sustaining of the spell, it will fail. The spell cannot be resisted.
By the description, it sounds like he could take a free walk into the tunnel as long as he was sustaining it. I would collapse the tunnel behind him, because it is “between” the two casters, so it cannot be behind the principal caster. Well, for that manner, could the auxiliary caster enter his own teleport tunnel? The rules don’t specify, but I have always assumed that they couldn’t. What do you think?
For those of you not familiar with the Book of Wishes, it is only currently published in hard copy. It has been re-edited, but the layout work has not yet been done to convert it into a pdf. Since the book has a lot of art, the layout work is pretty intense. We hoped to have it on our distributors’ sites reasonably soon.