My ISP offers 2 DHCP IPs to each customer, so I wondered if it was possible to
use both of them and rotate, but on 1 box with 1 eth card?

My Linux 2.4 kernel box with iptables is used for a NAT box. It connects to
the ADSL modem on eth0. No pppoe is used. (My home network is on eth1 which
connects to a hub)

Example: Something like opening two telnet clients (on the linux box) and one
is "from" one address, the other is "from" the other.

Would virtual ethernet adapters allow this?

eth0 being 1 address
eth0:1 being the other? (somehow routing through the "real" eth0 to the ADSL
modem and getting the dhcp address)?

How would I set this up? (how to get the dhcp for the second address?)

How do I choose which is default for the NAT'd boxes behind eth1's hub to go
through? How do I choose which is used in applications on the localhost linux

I was just wondering if this crazy idea is even possible. Any tips in the
right direction would help.