Oooooh, very interesting question. My nipples are hard.

(Spoilers)

If you take into account the story of Saint Shiva, the Iceheart Shiva is not the actual Shiva. So in theory, Iceheart's Shiva is only one Shiva. But what if the dragons interpreted Shiva a different way? Well, that'd create a different Shiva. None of these primals are established beings, they're the interpretation of the summoner. So in theory, two of the same primals could not be summoned at the same time. But an altered interpretation of that primal? Hmm, now we're getting metaphysical.

I'm a little torn on this, however. That basically means that absolutely anyone can summon anything at any given time with the help of the Ascians. Unless, of course, the Ascians are the ones who are dreaming up these primals because they know that they already exist in the aetherial realm.

I think the Ascians are behind these beings far more than we give them credit for...