Can I run an XP computer on 2 networks?

As time goes on, we're using our FTP site more and more heavily which occasionally has a really detramental effect on our office internet connection. I have a potential solution, but never having done something like this before, I'm not sure if it's viable.
 
The owner of the business lives next door but his computer running Windows XP is hooked up to the corporate network, (but in another property - another property with a seperate domestic BT phone line.) My thinking is that we could hook his PC up to a separate domestic internet connection and put the FTP server onto his machine.

Now my question is, if this machine is using a separate internet connection, can it still sit on our corporate network through another ethernet card?
 
Another potential benefit of this setup has occurred to me, but again, I'm not sure how viable this is. If the office internet connection failed, could his internet connection be shared with the server and act as a rudamentry backup internet connection?