double NAT with two cisco PIXs

hi,
can you please explain how to do double NAT with two Cisco PIXs.

Internet LAN(untrusted) LAN (trusted)
—— ——-
50.5.1.2 |PIX | 192.168.1.0 |PIX | 192.168.1.0
—— ——-

this is how i want to configure the network, i don’t know how this should work but if i can get this explained, i can configure.
any help would be nice
thanks