It's too risky as if you log in to your booking when your MileagePlus account is open ( logged in ), during booking the flight, viewing it via manage my booking, or checking-in on a web browser or the app; the system would auto-assign the MileagePlus number to the traveler matching the name of the traveler.

If you want to use them and credit miles to another FFP, you need to be super-cautious when using the UA website.