Unable to use co-authoring with Word

I'm attempting to use co-authoring with Word 2010 and SharePoint 2010.

I have all the settings in place (Office Web Apps installed, feature activated at site collection, Require Check Out turned off in doc library, and using .docx files).

I am using two separate machines with two separate user accounts for testing.  When I open the .docx file with "Edit in Microsoft Word", it opens for the first user, but if I attempt to then open the same .docx file with the second user using the same method, I receive the error "File in use; The file is locked for editing by (User 1)"

I have search the interwebs and the TechNet forums and I am unable to figure out why this is occurring.

I have reviewed the document here, and have troubleshooted all steps to no resolution.

If anyone can assist, it would be greatly appreciated!  :)