There was a lengthy thread I started about this very topic waaaaayyyyyyyyyy back when, probably in an earlier incarnation of this website (with a different logo, software engine, and ISP).
That is not to be dismissive, or say "Go use the search engine and don't bug me!". Rather it was, and remains, an essential element of usability, and the human factors side of stompboxes. It is also probably why the 1590BB will never die; because it provides an opportunity to stick a pair of stompswitches a reasonable distance from each other.
The inter-switch distance has a few aspects to it. One very obvious one, as you have pointed out, is "precision-on-the-run" - the degree to which the user can quickly, reliably, and with minimal attentional requirement, aim for one switch and hit it. Depending on the box, though, sometimes you want to be able to hit two switches at once....with the proviso that you can still hit one at a time when you want to. A little trickier in terms of planning.
Of course, if you plan for two switches on a 1590BB oriented with the widest edge facing you, that reduces the space between your foot and where the knobs/toggles are likely to be, so the temptation is to have the switches as close to the front skirt of the box as possible. Not necessarily the wisest move. First, there is a risk of the box flipping over when you go to hit it, unless it is VERY well secured. Second - and this is a personal bias, NOT a proven fact - I think the lifespan of switches is shortened by stepping on them more often from something other than 90 degrees from the floor. When switches are tooclose to the edge of the box, there is a tendency to rest the foot in a way that pushes the switch down at a less piston-like 88 or 84 degrees, stressing the switch chassis over time. But like I say, that's my bias, not an industry-wide maxim. Whether you buy into it or not, it is probably unwise to plan around the switches snuggled right up against the front or side skirt of the pedal. That can end up wasting internal space, but if it increases usabilty it is a reasonable forfeiture in my books.
HHE's question about shoesize IS just as realistic as it is tonguein-cheek, though. It is certainly worth your while to consider how wide your shoed/booted foot is, and be guided in spacing by that. After allm, it is not just your foot, it is the "actuating bio-mechanism".
use of two stompswitches also imposes the challenge of figuring out where the battery will go, though most will situate a battery holder between the two switches along the front skirt, if they use a battery.