I have 2 official viv stacks and 3 other vivs stacked on top of each other. I use heatmats for all my snakes and have never had a problem with heat from the bottom vivs effecting the higher ones. The only time I did have problems is when one of my stacks was fitted with heatbulbs. Then the heat from lower vivs did effect the higher ones and I really struggled to maintain temperatures. Having said that i had all 3 bulbs working off 1 stat so maybe I would have had more success if they had all been individually stated.
I have a stack of 4 vivexotic 4ft vivs ontop of each other, you do not need gaps but I use heatmats, there really is not much of a temp difference from the bottom cool end viv to the top, like 1c according thermometers, heatmats are all on separate stats.
I also got mine from swell, come very quickly, sent me one of the vivs in the wrong colour but they quickly sorted it out.