It is entirely common for there to be a single award seat available. If two different accounts try to book it at the same time, only one of them should ticket successfully. I wouldn't try it -- even if it appeared to work, you may end up in a situation where UA discovers the situation later and fixes it by rerouting or delaying one of the passengers.

The best advice for trying to get two premium-cabin seats is to be patient and diligent -- keep checking inventory regularly to see what might open up, and if you see something you like, don't delay, because it may not be there later. The fact that you have a backup itinerary ready to go means that you can afford to relax -- the worst that can happen is that you'll spend longer in transit than you'd prefer. Heck, there are plenty of people on this forum who would intentionally route through Europe in order to maximize the amount of time they spend up front.

Have a great trip!