As TD says, it's a matter of getting lucky and getting paired up with someone who has the same qualifications as you.
IF you get a JCAB ATPL partner then you will save about six weeks of training. You will not have to do the first two weeks, which are the air law and radio classes and exams. Having avoided those you will join your classmates for the 767 type rating course. So you'll have about four months of ground school, FBS and FMS sims, culminating in your 767 type rating check ride. You'll then miss out on the next month of 'training', during which the rest of the class will do the additional sims ready for their ATPL check. After that you will do the two weeks of airport briefings, followed by your OJT (line training). Whether you are able to 'jump ahead' of the rest of the class and do the airport stuff and OJT is just a matter of guesswork. You may do that or you may have to wait a month while they catch up with. It's unlikely if the others are FOs and you're capts but never say never here
If your partner does not have the JCAB ATPL then you'll still avoid the air law and radio but you'll be stuck as his 'sim buddy' for his ATPL 'training', so you'll save a whole two weeks
