connect to 3g internet connection (shared by iPhone) while connected to local network (wifi) without internet

connect to 3g internet connection (shared by iPhone) while connected to
local network (wifi) without internet

currently i need to connect to a local network that doesn't has any
internet connection
let say this is the rough idea of this local network :
router : 192.168.1.1
computer a : 192.168.1.2
computer b : 192.168.1.3
...
computer n : 192.168.1.x
my computer ip in this network : 192.168.1.23
since i need a internet connection, i try to share my iPhone 3g connection
to my computer. after that, what happen is my computer got 2 ip (first one
is the one that i got from the DHCP wifi network, the other one is my
computer ip to connect to my iPhone).
the problem is, i can't browse anything or connect to the internet after i
come to this state (2 ip addresses), it seems my computer always try to
find internet connection through the local network wifi instead of my 3g.
can i got my computer connected to this local network through wifi while i
can using the internet connection through my iphone 3g connection ?
the iphone 3G connection is shared via personal hotspot.
help me. any link of tutorial or help will be very appreciated
btw, my computer is osx 10.7, hope can get some configuration that i can
use in this os