The alias command is for "DNS doctoring" (have inside and outside interface) or "Destination NAT" (inside, DMZ, outside interface).
Even you use alias command, you still can access the public ip address from the inside for sure. Only when you do DNS enquiry, the PIX will translate the public to your private ip.
If you want "Destination NAT", you need three interfaces to make it working.
Please check in following URL:
http://www.cisco.com/warp/customer/110/alias.html