I also am presently seeing that when I search for 2 tickets - shows as "Not Upgrade Eligible" at the first Trip Summary page after choosing my outbound and return flights, after I go through the "Enter Passenger Information" screen and get to the Review before purchase screen it switches to Upgrade Eligible.

In the case of searching for 2 tickets the absence of the 2nd passenger's SM number appears to be causing the "Not Upgrade Eligible", once that gets filled in on the pax info screen we're Upgrade Eligible.