Double and triple stacked just refers to the physical size of the pill.
Someone, somewhere, had must have thought something along the lines of;
"If I make the pills twice the size then the idiots that buy these things will think they have twice as much MDMA (active ingredient in ecstasy - if you're lucky) in them"
Like most advertising gimmicks this is complete bullshit.
You could make a pill half the size of a normal pill and still stick more MDMA in it than the average pill contains. This is because the average pill weighs about 300mg's. If you get more than 100mg's of MDMA in your pill then you're damned lucky. The rest of the weight is made up of inert binders like ground up china (I think) that help the pill stick together. These binders are necessary to prevent the pill crumbling to powder, but they don't make you high.
If you have yourself a triple stacked pill that weighs (for example) 800mg's, then the odds are it contains the same amount of MDMA as a normal pill, and 700mg's of inert binder.
In other words the fact that a pill is double or triple stacked does not mean that you have more MDMA. It just means it is harder to swallow.
I bet the guys that make these things laugh every time they hear of someone plugging one of their pills