I actually did research on this subject (one-handed ordination) in the early 1990s when it first became a hot topic. Not only have the Eastern Rites always used one hand, but the Roman Rite also did for several centuries, and then switched back. I went to The Catholic University of America library and found the original editions of the Pontificale Romanum from those times; CUA had a huge collection of them going back a very long time, and I found where the right hand dextram was indicated. Now, the article posted by Matto simply dismisses this as irrelevant by asserting that Pope Pius XII decided that it was two hands. No, he was simply indicating the point at which the essential matter+form were combined to confect the Sacrament, which simply happened (in the current practice) to be two hands. Nowhere does he state that the ordaining bishop must use two hands for validity. This is effectively like saying that if a Roman Rite bishop used an Eastern Rite formula, it would be invalid ... which is total nonsense.
To this day, Bishop Kelly holds all the Sacraments conferred by Bishop Williamson to be invalid .. and those priests ordained by Bishop Williamson. That would basically render the entire Resistance line of bishops and priests invalid. They re-confirm people who had been confirmed by Bishop Williamson (I don't even know if it's conditional).
So, prior to Pius XII, there was some disagreement among theologians regarding the exact point at which the ordination took place, where the essential matter and form were combined and the priest became a priest, since there were a number of laying on of the hand or hands. All he did was say "here's where it happens." He was not making it so, but simply indicating where it has always happened in that Rite.
So an analogy might be that the Roman Rite prescribes the use of unleavened bread for the Mass, whereas the Eastern Rite has long used leavened bread. Now, for a Roman Rite priest to use unleavened bread unless out of necessity would be gravely illicit (if done intentionally), but it would nevertheless be valid. There are many stories of priests in prisons and labor camps who used whatever bread they could get (usually leavened) to offer Mass in cαмρs. They often also used very abbreviated consecrations ... since the Missale Romanum was not available there, obviously. In the case of Archbishop Lefebvre in 1976, since when he did ordinations all by himself in those days, he would go from ordinations to the sub diaconate one day to ordinations to the diaconate the next and finally ordination to the priesthood, so it was probably done out of habit because the previous ordinations were done with just one hand. So there was no sin there either even if he was objectively out of compliance with the rubrics. Priests mess up all the time, unintentionally, when they get confused by the rubrics.