Not sure if you can do two back to back but I don't see why not. Although I'd do the numbers first because since tickets on the regular rate get cheaper the more you buy, it may be a better deal the regular non-discount way for 8 days. We stay for 8+ days too and have added the water park and more option to get an additional 4 days worth of activities besides the parks for about $30 per ticket (great deal!). Four more days in the parks would be great though!
Some things I learned first time I used these tickets...
1) When you get them from the MWR office they are not tickets per se but just vouchers. The vouchers have serial numbers on them that CAN be entered into MDX so you can set your Fastpass+ up ahead of time.
2) When you get to the park, you have to go to a ticket sales window at one of the four main parks to activate them. They are useless until then. I found this out the hard way. We had added the water park option and went to Typhoon Lagoon the first day thinking we could activate them there. No go. We ended up spending the better part of two hours riding busses to get to the ticket window at HS and then back to Typhoon after activating. This time, I am going to hop the ferry over to MK from Fort Wilderness the first night right after I get my camper set up just to get it out of the way.