How wide should the framed opening be to accommodate double shed doors?
You'll add a list of things to get your R.O.:

The width of both door slabs together
The width of both side jambs together
1&quot; gap +/- for shimming the jambs plumb inside the framing (1/2&quot; for each side)
~3/4&quot; for slab gaps (1/4&quot; each left, center, right)

The slab gaps may need to vary from 1/4&quot; depending on your hinge and latch hardware. 3/16&quot; (9/16&quot; total) is more ideal, as it makes for better latch engagement, but requires fairly precise carpentry.
Of course, you can frame your opening and build the doors to fit it instead. Just work the same numbers.
Assuming standard outswing doors and 3/4&quot; jambs, your 36&quot; slabs would need:
1/2&quot; + 3/4&quot; + 1/4&quot; + 36&quot; + 1/4&quot; + 36&quot; + 1/4&quot; + 3/4&quot; + 1/2&quot; = 75-1/4&quot;

If, however, you don't use floating jambs, disregard the jamb thickness and the outer (1/2&quot;) gaps. You'd then simply have:
1/4&quot; + 36&quot; + 1/4&quot; + 36&quot; + 1/4&quot; = 72-3/4&quot;

An astragal doesn't usually add to r.o. in a simple shed scenario. It simply mounts to the face of one slab and laps over the opposing slab. There's really no need for it to be mounted to the edge of the slab. If you do want to use an astragal between the slabs for more stable latching, take that width into consideration.
Also, you mention that there's no space needed for your hinges. Even if you're using strap hinges you don't want the slabs tight to the jambs. That would be a recipe for binding and noise.