Combine multiple ETC on one passenger PNR possible
Good news! Seems like UA changed their policy about using multiple ETCs on a one passenger PNR. I contacted Customer Care about it, since I couldn't find anything in the ETCs Terms and Conditions. Their reply was:
"Yes, you do have the ability now to combine multiple Electronic Travel
Certificates for one reservation. You would need to contact a
reservations agent to have the certificates combined into one
certificate. You will be emailed the new Promo and Pin code. Once
received you can apply the new travel certificate toward future travel
on united.com."
When I called the Premier Priority Desk they just completed the booking for me over the phone and applied both ETCs without a problem. But I'm sure you can also ask to receive a new ETC with the combined value of all your ETCs. However, I doubt that that you can combine multiple ETCs issued on different last names but I will leave that for somebody else to try.