Just to provide a counterpoint - I took both 8 hour SE exams on the Friday-Saturday in one exam administration. I took the exams in Chicago because I "needed" the SE in Illinois and at the time, Illinois was not granting reciprocity for the SE taken in other jurisdictions. I drove 8 hours to Chicago on Thursday and back home on Sunday. I wanted to put in more time studying, but I only got about 200 hours of exam prep specifically for these exams. I have posted about my prep, my circumstances, and my thoughts about it in other threads, so I won't reiterate all of that here.
What I wanted to emphasize in this thread is that I dedicated the entire weekend to the SE exams. Staying in a hotel away from home may have been helpful for me. I restricted my diet and fluid intake over that weekend and adhered to a strict bedtime with the help of a sleep aid. I limited tv watching and reading over the weekend. I did not access internet or e-mail over the entire weekend. If anyone reading this wants more of that sort of nitty gritty detail, PM me. My point being, it doesn't take that much more planning and discipline to take and pass both exams in one administration. But the real issue here, in my opinion, is whether or not you are the type of person who can make the sacrifices to take both and whether or not you have the discipline to stick to the schedule and not allow yourself to be distracted over that weekend.
In the end, I felt that the lateral was easier than the vertical and I felt really good about my performance on both days. I found the experience was challenging, but invigorating and I'm happy as hell that I took and passed them both in one weekend. It is realistic for some people to prepare for and successfully execute the examinations in one administration. It's up to each and every one of us to honestly assess our individual situations and determine whether or not to attempt both exams in one weekend.