2 pix 1 public ip

hi, our company was recently aquired by a large company. and for us to be able to integrate to the companies mpls, we have been asked to change our ip adresses. so we have set up a backup router and pix with the new network to do some testing before we migrate everything. but on the pix part we have the same settings on both of them but different networks and they both are nat outside to the same public ip xxx.xxx.xx.3. the network on the main pix we are able to get to the internet but the new network we can not. any sugestions as to why? if we nat the traffic to two different public ip xxx.xxx.xx.120 this works.