Routing 2 incoming networks into 1
 

My question is hypothethical. If I have two modems that can be connected to PC via NIC and one small home/office network can I use both modems with one linux box, acting like a router? How many NICs do I have to possess? eth0 and eth1 for the modems, and eth2 for the inner network? And can that routing be done via iptables? I want the PCs in the network to recieve net flow from the gateway (the linux box), which one recieve traffic via the modems.
Detailed explanations for that situation are welcome :)
Replies like 'bye one modem with larger traffic channel instead two modems' are not welcome :)

The subject was described too short. Of course you can use 3 NICs: eth0 & eth1 for modems and eth2 for internal network. But the rules of routing are unclear - so it is a point you should think about and clarify firstly.
But in general such situation is a matter of traffic shaping and advance routing. So read some about it. For instance see this link.