for outbound traffic, it shouldn't matter how many layers of nat deep you go as long as the gateways are configured properly for each 'layer'
basically, the NAT machines should have two lan cards, an external interface and an internal interface, then the NAT happens between them
nat1 server (gateway = 'modem's internal ip')
+-nat1 lan (gateway = 'modem's internal ip')
nat2 server (gateway = 'nat1 server's internal ip')
+-nat2 lan (gateway = 'nat1 server's internal ip')
the nameserver IP should also be the same as the gateway ip
this is essentially how a DMZ is created
in a DMZ situation, public facing servers (such as web servers) would run in nat zone 1, then the actual company lan would be nat zone 2
the only real issue is when you want to forward INBOUND traffic into a deeper layer of nat, you would forward the necessary port from the modem to nat 1, then from nat 1 to nat 2 and then nat2 forwards the port to the actual machine.
hope this helps