Wow, just ran into this issue last night. I booked an award ticket from mom's account, went to find the same routing to book from dad's, and couldn't. The second seat had disappeared. Called to cancel mom's ticket (to book a different routing) and was told that 24 cancel policy doesn't apply to award tickets. That is incredibly unfriendly!

To the agent's credit, she did allow me to swap to the other flight without a fee since I had booked it literally 5 mins prior.

So be careful. Yes, the hold could be helpful, but I think I'd rather have the 24 hour worry-free cancel policy like United has. (And an unlimited cancel policy for 1K's!)