This is true, but YMMV depending on how dark the color is, and how much tint is added. If it's a beige or taupe color, you should expect there to be about 3 ounces of colorant in there, give or take. These usually go in to a light tint
base sometimes referred to as a "pastel base", "Ultrawhite", or "Base 1". There will generally be enough room in the top of the can to double the formula.
However, this is where it gets tricky. For a dark or saturated color, like red, black, or deep brown (plus many others), a deep base is required, and up to 12 ounces of colorant may be added to form the needed color. An additional 12 ounces of colorant would overflow the can by a long shot and simply substituting a larger can without adding more tint base would seriously mess with the paint's properties -- colorant never actually dries as it relies on the binders in the tint bases.
A custom-mix black (meaning, one they shoot at the store rather than coming straight from the factory pre-tinted) is simply 12-13oz of Lamp Black (NovoColor Colorants) in a Deep Base or Base 4/Base 5 depending on brand (they all have different terminologies for their tint bases).
I work in the paint department at Lowe's and mix paint/color match all day. And yes, I use the machine (spectrophotometer) to do color matches, but usually only as a starting
point. I will generally dispense only half of its recommended formula and do the rest by eye as the machine tends to error on the dark side. It's hard to lighten paint that's too dark, but much easier to dispense the other half of the formula if need be. It's also easy for
me to look at the resulting color and say "this needs half a shot of green to get rid of that magenta hue".
Anyway, yes you can generally double-tint, but only if it is a fairly light color to start with.