I can agree with what Prostringing has said. And here's my explanation:-
You (starting) clamp on the outside of 2RM at the throat. You then pull tension on 1 RM from the throat. This means you pull tension on 2 strings, 1RM & 2RM and full reference tension is not applied to 1RM. So when you re-tension 2RM (to remove the starting clamp), you're simply pulling tension of the same 2 strings you tensioned at the beginning.
From what you've said to prostringing, the method you used means that 1RM will be at reference, while 2RM will be less. This isn't right as what you did is double pull i.e. pull 2 strings at once which will give a lower (required) tension. In fact you did this twice.
Head IG Prestige MP; USRSA MRT, Wimbledon stringing team since '04; Babolat stringing team '10-'14. Twitter - @therqtstringer