I'm a POS and do this frequently (and have posted about this in the past). Here's the lowdown:
- Buy first ticket under your regular name (Mr. Big Bit).
- Buy second ticket (preferably on same PNR) as Mr. Extraseat Lastname (Mr. Extraseat Bit).
Select the two seats together. UA will NOT allow you to put your FF# on Extraseat's record. At the gate, give the agent both tickets when boarding. If he/she is confused, explain what you are doing PER UA POLICY. Fly the flight and keep both boarding passes and tickets. You will automatically get all applicable credits for your first passenger. Take a copy of your tickets and boarding passes and submit to the MP fax # (the one to get missing credits). Tell them what you did and that you want RDM credits for the second seat, as per their policy.
They will only give you RDM credit for the extra seat at 1X, regardless of status, but it's better than nothing! You will not get EQM, BIS or credit towards 500's CR-1's or MM status on this second seat.
I am flying this way LAX/LHR next week. Two Y tickets at $550 each is a lot cheaper than C at $6-7K.
Apparently, the other group of people that frequently use this tact are Cello players who don't want to check their instrument as baggage.