Why mix dry and fresh yeast?
When both are used, the fresh yeast is for "added flavor", the dry for leavening power.

Another author who does this is Nancy Silverton.

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Some bakers believe that fresh yeast (a.k.a. cake yeast or wet yeast) gives a more yeasty flavor to the final product. I don't think it does.

It's also more perishable, (sometimes) more expensive, more difficult to find (and when you do, it may be near its expiration), more challenging to use, and less reliable (re: reproducible outcome).

When I encounter a recipe that calls for it, I convert to instant yeast using this old, but reliable chart: http://www.theartisan.net/convert_yeast_two.htm

If you search The Fresh Loaf for instant yeast, you'll find more information (that may or may not be reliable).