We did this one day on our recent trip, except we used one in the MK in the morning and a second at AK in the afternoon. We received a very hard time from the CM at AK on using the second voucher. When you redeem a voucher for FPs, they swipe each park ticket to ensure that it is a valid ticket. In doing that, the fact that you are getting the first FP of that day "attaches" to your park ticket. Therefore, when we tried to use the second voucher, the system initially did not allow it. She eventually called a supervisor over who authorized it and overrode the system, so we were able to get the second set.
Therefore, I think you might have a hard time getting the second set in a day, but given that you have 6 vouchers on a 4 day trip, I sure would try! If the CM can't/won't do it for you, make sure to ask for a manager. Be very nice and hopefully they will override it for you.
Those FP exchange cards are awesome! They saved our trip for us on days that wound up being unexpected 10s on the crowd calendar.