Sounds like this has nothing to do with two monitors as it does with two users on two workstations.

That being the case you'll need a client-server technology of SOME sort so you can sync the excel document from two remote locations. Without looking too much into it I instantly start thinking about:
1. There's probably a third party addon for Excel somewhere. Look for Excel sychroniziation. I came across this (http://www.synkronizer.com/)
2. Don't feel like paying? Then build a VBA synchronization routine. Have it write to a common database or XML file that can be polled/refreshed.
3. Sharepoint I think does this kind of stuff, but I think that's a little out of the scope.

The problem is I don't think Excel natively supports keeping a remote excel spreadsheet in sync on two different workstations, but I could be mistaken considering I don't have access to anything past 2000 right now. Just my brain dump as far as that goes. HTH