I believe NAT (Network Address Translation) is not what you are seeking, however, it may not be possible to do **exactly** what you want either way. The closest thing I know of for using two or more networking cards to get more bandwidth/throughput is by using the Linux kernel's 'bonding' driver, and optionally, the 'ifenslave' tool, included in the kernel documentation. It isn't that difficult to set up, but what you can achieve with it will vary depending on your network topology & equipment.
For complete details on the many modes of operation you can achieve using the 'bonding' driver, please read carefully the file in the kernel Documentation folder, located as follows:
It's a very comprehensive document.
If this doesn't appear to be what you want, post back, and maybe someone else will have another idea for you.
Here's some further reading:http://serverfault.com/questions/267...bonding-driver