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.