On the subject of Attunement, I always thought of it as being like it's described on page 45 of LiaB
"Between the volcano link-up and his three recent levels (two from traps and combat, one from all of the city rebuilding), Sizemore was now a greater Dirtamancer than he ever imagined he could become. He had not actually crossed the threshold to Master class, but that could happen any time a caster gained powerful new insight into his discipline or major class. The body analogy wasn't quite enough to push him past that mark; he was still missing something. But for the first time in his life, he felt pretty sure that he would get there."
Wanda was the only one we've actually watched attune to a 'Tool, but she knew in advance that she was going to attune to it thanks to Predictamancy and Fate. She was already in the right frame of mind, if that's an acceptable way of putting it, to attune. Someone else - especially if they have only recently stumbled across their 'Tool unexpectedly - might take more time to attune. Hell, it's right there in the term we're using. You have to attune, to adjust, to be changed either slightly or fundamentally, before you can truly access the power of an Arkentool.
As to whether more than one unit can be attuned, I think page 29 of LiaB
lays it out nicely. Lady Sylvia's discussion with Ossomer as she describes picking up the Arkenpliers after the fall of Ansom:
"Lady Firebaugh has said as much, and I have seen as much. I have held the Arkenpliers, did you know? Have you?"
Ossomer nodded solemnly. "I have. Each Prince of Jetstone held them at least once, in hopes of attunement."
For the first time, Sylvia smiled. It was only the barest tightening of the lips, but it made her seem present in a way she normally did not.
"Do you remember the sense of the divine? Did you feel that your hands were in contact with something greater than the world?" she asked. She turned her head and looked into the featureless distance. "The Arkentools were used to build the world. I wielded the Arkenpliers, and had the feeling that they could build more worlds than only this one. Their power was primal, undeniable, and very strange."
She clearly felt something that neither Ossomer or Ansom did, but didn't instantly attune to them. Maybe because Wanda was fated to attune and that blocked Sylvia from doing so, or maybe because she needed time to attune properly as suggested above, while Wanda was already ready to attune at the first chance. But this suggests to me pretty strongly that more than one unit can
be ready to attune, even if only one unit can be an attuned Wielder at a time. Sort of like the Potentials from Buffy the Vampire Slayer
- they're ready, and the already have some level of power that most normals don't, and when Fate calls they'll get the full measure of power. But they just don't get to use that power fully yet.
But of course that's just my opinion.