, I've seen lots of multi-homed servers. (We currently set up most servers we do with three NICs) But I haven't seen too many PCs with dual NICs in quite a while.
I have heard of doing that in the past in rare cases where the goal was to increase available network bandwith by concatenating multiple NICs. But most of that came to an end with the advent of affordable 100Mb and faster NIC cards. However, it was mostly done, as others have already noted, to provide separate networks for security purposes; or to move bandwidth hogs (video conferencing, etc.) onto their own 'channel.'
Be interesting to find out why your friend's network was set up that way. General network security can be better (and more economically) provided by your standard WAN/LAN NAT+SPI router/switch combo. So I'm guessing it was done for performance purposes.
I'm curious. Do all
his machines also have something like a heavy duty analysis or financial app constantly running in the background?
I know some brokerage-type businesses who do something similar. They use multiple NICs in their machines to better accommodate those apps since they need to be kept updated in near real-time. (They also have a dedicated T1 on the WAN side just for those apps as well.) But that's a pretty odd duck client requirement.