If this is a UA return award remember that you can't cross both oceans according to the rules. (The website sometimes lets you book it, but if you need to make changes agents will spot that it isn't allowed and make you change it.)

If you don't need a stopover then you can change the award to 2x one ways to cross both the Atlantic and the Pacific for the same number of miles as a return, but that can cause problems with availability if you are already holding tickets.