I know that it's possible to mix ASP & ASP.NET 1.1 in the same
website/application so long as they are in separate frames (iframes). This
also holds for mixing ASP & ASP.NET 2.0 pages. But what I haven't tried
is mixing ASP.NET 1.1 & ASP.NET 2.0 pages in the same website/application
but each page in different iframes.

I don't think it's possible to to have both ASP.NET 1.1 & 2.0 pages in
the same website since IIS defines which ASP.NET version to use. Does
anyone know a way to make IIS use ASP.NET 1.1 for certain pages, and
ASP.NET 2.0 for other pages?

Probably one can host the ASP.NET 1.1 website, and link in the iframes
of ASP.NET 2.0 pages from another website in the same IIS server. But
that's just looks clumsy.