2 HTML documents in one

Dear all,

I've been looking at this Web site for sometime and wondering about its 
validity, both from a coding point-of-view, and also from an 
accessibility one too. If you look at the source code there are 
actually 2 HTML documents on the same page - I've never seen this done 
anywhere else and was wondering if it is acceptable or not. The 
technique is actually utilised quite frequently across many of our 
colleges web pages. It doesn't validate, but not for the reasons I 
would have expected.

http://www.chester.ac.uk/performingarts/

Any thoughts?

Phil