Two questions:
1) Can you have duplicate passes even if they are the same pass, in this case two active Premium Annual Passes at the same time? I'm assuming yes but figured I would ask because that's a little different than having separate parks AP and DisenyQuest AP.
2) What if you want the two passes to run consecutively? First PAP is purchased 12/1/12. Runs through 12/1/13. How close do you have to be to 12/1/13 in order to use a voucher to extend to 12/1/14? Can the voucher be redeemed in November '13? August '13? June '13?
# 1 Yes you can.
#2 Your pass is good for 365 days from the day you redeem it, so for it to be good until 12/1/2014, you could not redeem it before 12/1/2013 because you would get more than your 365 days. If I am understanding your question.
If you redeem a pass in June of 2013, it will expire on the same date in June of 2014, redeem in Nov, expires in Nov of the next year, etc.
The rule for renewals would not affect this situation, as you are not renewing an existing pass, you are redeeming a voucher for a new pass, so the 365 day timeline applies.
Now if you are not going to be at Disney world to redeem the pass at the latest date, you can redeem it earlier but the 365 days begins then.
For example if during the special time period you buy two PAP's and you redeem Pass A on Dec. 1, 2012, it expires on Dec. 1, 2013
If you save Pass B/voucher and you take a trip in November and you know you are not going back before Dec. 31, you can go ahead and redeem it right before you leave, and it would be good until that date, 2014.