I wouldn't personally recommed trying to get two live inputs through Firewire or USB for many reasons.

One will work just fine, but more than that will give you trouble for sure.

Do you need the inputs simultaneously? Or can you put them on a switch?

If you do need more than one input simultaneously you really should use the Active Silicon LFG-4 card for composite inputs.....this will give you 4 composite inputs per card (maximum two per server)

If you need SDI or HD-SDI you should use one or two of the new Active Silicon Phoenix SDI cards as your needs require.....this will give you two SDI or one HD-SDI per card (also maximum two per server)

You could use these cards together as well....ie (x1) LFG-4 and (x1) Phoenix SDI.

Hope this helps