Server with 2 NICS

If I'm understanding you correctly, you want to create load balancing from a single router to two separate NICs on your server.  It's really not a simple process, and is illustrated here:

http://blogs.technet.com/b/josebda/archive/2010/09/03/using-the-multiple-nics-of-your-file-server-running-windows-server-2008-and-2008-r2.aspx

Also, I'm guessing your router only has 1 port for internet connectivity?  Without some networking equipment between your server and router, you're only going to be able to plug-in one of those NICs into the "internet."

manage devices on both subnets from either subnet

The purpose of separate subnets is ... network separation, so I'm not sure your concept would be considered a best practice.

Hopefully this sheds some light on your situation. :)

The problem is that I have a client with over 300 computers. They have 1 file server, but it is used only very lightly for minor file storage, and printing. I would like to split them up into several subnets. Would a level 3 switch be a better solution?

Are you having throughput issues?  What speed is the network?  100Mb, 1Gb, or 10Gb?  Unless you're reaching your throughput capacity on the NIC, there's no point in splitting up traffic between two NICs going to the same server, since both NICs are going to use the same compute resources.

I realize now that  a better solution would be a true router or level 3 switch.