The technical reason why you can't do this is that UA's reservations system enforces constraints for a multi-passenger PNR which require that every passenger must have identical flight numbers and fare codes.

Some of the things you can't do include:

• Have two different flights, one for one passenger and one for another passenger, on the same record.
• Have the exact same flights booked, but one passenger is on an W fare and the other passenger is on an L fare. (This is why buying two one-passenger itineraries is sometimes cheaper than one two-passenger itinerary -- a flight that's Y9 B9 M9 … W2 V1 will book into W for two passengers but may book into V for one passenger and W for the next passenger if booked on separate PNRs.)
• Have the exact same flights and fares booked, say both K fares, and then upgrade one traveler. (This would leave you with an R fare and a K fare on the same itinerary, which is a no-go.)