Error 404 not found when put 2.0 in folder 1.7

[eluser]Unknown[/eluser]
Hi everyone
I got problem when I use multiple web by put CI 2.0 in the folder that use CI like this

I implement with CI 1.7 as
www.domain.com/webci17/webci20
domain.com use normal php code not use framework.
webci17 use CI 1.7 to implement
webci20 use CI 2.0 to implement
I can access to www.domain.com/webci17

but when I acces to
www.domain.com/webci17/webci20

It's will redirect to 404 not found page
I check by edit index.php in webci20 to any text it's change display as I change but when change back to default CI index.php it redirect to 404 not found page again anyone can solve this problem.
Thank you.