You're fortunate to have an assistant. I don't have that luxury, so I've had to find another way to do the long seams. What has worked best is to lay a 4' x 8' piece of masonite on the floor and put a 4' x 4' piece next to it, so I have a work surface 12' long. Then I put one half of the tarp down and pull it taut, fastening it at each end with weights and pieces of painters' tape. I use a long strip of blue painters' tape to mask the edge of the 1" edge that will be bonded, so I can apply the Primer 94 with a small paint brush. The throw-away foam brushes work fine. Then I apply the 9460 VHB tape to the primed area. Because the cuben is held flat, one person can do this quite easily. Then I remove the blue masking tape. Next, I prime a 1" strip on the other half of the tarp in a similar fashion. Then I put the primed edge of the second tarp half over the taped edge of the first half and fasten it in place with weights and tape at each end. The backing paper is still on the VHB tape, but everything is lined up for bonding. Then I carefully start peeling the backing paper off at one end, but instead of pulling it straight back in line with the edge, I pull it at an angle to one side, so I'm pulling it out from between the two sheets of cuben, which are already in contact. To get it started you need to move the weights and lift one corner of the top piece. Once a 3" end of the backing paper is free, you can roll the bond and put the weight back. Then roll the entire seam as you pull the backing out, with the roller following an inch behind the backing as it comes off. If I did have an assistant, I think I would still use this method, with the assistant pulling the backing paper, so I'd have two hands to hold the cuben down ahead of the bonding area and roll it tight immediately after. I use 1" wide tape. A single backing strip is easier to pull than two, but it can be done.
It's possible to prepare the cuben by trimming the edge with a soldering iron along a metal straightedge just before applying the primer, but using a cutting mat and rolling cutter is just as easy. I have thought about a cutting mat that's 6" wide and 12' long, but I don't do enough of this stuff to justify the expense. (Besides, they don't exist.)