No doubt about it, it WILL post as 1 segment. Just went back through my records:
15SEP2005 NW 0525 NW 0525 T/T DCA - LAS 2,089 1 2,089
Note the * 1 * segment on this direct flight. To boot it has been well documented that if there is not R class available on both flights in the same seat you will not get upgraded. Welcome to the fun.
The above flight was actually DCA-MEM; MEM-LAS