Site1 and site2 don't have the same content.
Of course they don't have the same content, this is multisite module we're talking about...

that's why I only redirect the shop folder.
My interface is not Cpanel.
with my panel I can redirect full site or a sub domaine. but not a folder.
But you don't need redirects! You need to change the document root, nothing else.

Let me see if got this right:
you have site1 with some content in the root
you installed ZC at www.site1.com/shop
you have site2 with some content in the root
you want multisite at shop2.com/shop

is that right? If not, please tell me exactly what you have and what you want.