Originally Posted by orbitmic

In theory you can't and if/when the system finds that a person has two bookings on the same flight, it might cancel one of the two and you won't be able to control which.

It is perfectly possible that the system would not "spot" the double booking but it is not impossible either that it would. If you do not wish that to happen, it would be best not to have personal information (FF number, passport number, etc) on the booking so as to lower the risk of booking cancellation. Of course, you would need for both bookings to be on a single segment as otherwise, the no-show would cancel the rest of the booking.

For what it is worth and fully understanding that I should only answer my own question and mind my own business, I would mention that I think that even if it might work out, what you propose to do is not a good idea. It is really messy, not least in that it could lend you in trouble with the company in question which TA could of course be alerted about the no show. You would also both be wasting money, they on a flight that will be waste, and you on taxes that do not need to be paid twice. I would strongly recommend that either you tell them that this is not acceptable to you as a travel arrangement (if you think that this is possible) or ask them to pay up the difference. In this type of case, clean and transparent is vastly preferable.