im stil not verry sure what your trying to do exactly because your explaination isn't that informative.
your talking about lans modems, network cards and firewalls in 1 sentence etc...
it might help if you created a scematics of what your network looks like, whitch devices are where and what thair repsective ip's are...
from what i understand now your are trying to manage 2 seperate networks (lans), while there are still internet or vpn modems are connected to these ports as well as desktop computers?
if this is the case than general rule dictates that its bad practies, and you should seperate your internet modems from your local area nework by having 1 network card for the wan (internet) side, and one for the desktop (lan) side.
if you have more than 1 modems (internet connections) than you can deside if you use more than one network cards and modems, (prefered / easy way), or if you connect multiple modems to one interface (hard / dificult way)...
for the lan side in most cases 1 network card can suffice, but you can add as many as you like, (maybe one extra for a wifi network, or whatever wishes you may have... its up to you... \
you can read all about this at:
http://doc.zentyal.org