I assume that both have been confirmed as being activated with the same subscription LiveID.
Unfortunately, I don't have a clue what else you could try except that the few similar reports of a hub not seeing the status of the other PCs in the Circle were typically resolved by reinstalling.
What operating systems are on both PCs?
How are they connected? Wired or wireless and both on the same LAN?
You might try reinstalling one copy first and seeing if you at least get the "Your OneCare Circle" link/tab and the status of the other PC, even if it is unknown.
-steve