Yeah, "stacks" is, frankly, bullshit branding.

The dose of MDMA in a given pill is highly variable, from 50 to 120 or so (mg), discounting pills with little to no MDMA. "Stacking" doesn't refer to any actual part of pressing a pill. Making the MDMA is the hard part; then, you just put however much MDMA (and/or other stuff) you want in with inert filler, press the pill, and bam! Done.

Sometimes, double-stacked means a potent pill - closer to the high end of the common dosage curve than the low. More often than not, it means nothing at all except "I want to move these pills faster / charge more for them." In fact, I'd be extra suspicious of these pills; some people believe that they're more likely to contain meth or other drugs to make them more intense.

In conclusion, "stacks" on a pill are kind of like "dips" for LSD blotter: meaningless terms invented by dealers to advertise their wares as superior when there's no reliable way of actually comparing relative strength besides lab tests or ingestion.