Your question intrigued me, Peter, as I cannot imagine anyone cutting a tree fern in half - I hope you mean vertically. As no one else has yet tried to answer it I have just done a web search and cannot find any mention of this means of propagation. The RHS site:
http://apps.rhs.org.uk/advicesearch/profile.aspx?pid=171
refers to propagation by removing offsets. Perhaps this is what you have heard of?

Like Bulbaholic, I was intrigued by your question and found this site: http://gardening.savvy-cafe.com/how-to-split-up-your-ferns-without-harming-them-2007-03-16/ which seems to suggest you CAN simply cut some ferns very carefully in half. Notice it says that by tearing at them you can easily kill both halves!
Given the amount you have to pay for a tree fern, I wouldn't risk it, and wait to have an offset as suggested. I've lost every tree fern I've ever bought because of very cold winters so if you are succeeding in keeping yours alive and vigorous, you should be happy!

I would think the article Bertie refers to is for the perennial herbacious ferns rather than tree ferns. ordinary garden ferns can be split by cutting a clump up along a natural division. as for a tree fern I'd suspect not. Their growth habit is very different. certainly offshoots can be done this way.