Sometimes, United's web site shows 'phantom space' in the upgrade/award buckets, so one strategy is to call to see what the real-time information looks like and confirm with an agent. Another is to utilize the 24 cancel functionality by booking and then applying the upgrade (based on positive NC/NF space seen on the web site) and, if no joy, calling to confirm the said lack, then canceling, if appropriate.

Also, as both travelers are 1K, a concurrent strategy would be to book the travelers on separate PNR's.

Good luck!