As both bodies were Nicks he could in fact multi task using them and as it turned out the dwemer was more cautious than expected. Rather than stuffing all the knowledge within it into the head of whoever tried to read it the small cube only took out a single tomes worth of information at a time before it needed to be activated again. In a way Nick had to admit that it made sense as the dwemer wanted to protect it's knowledge from being lost so obviously it wouldn't do if the method used caused the person reading it to go braindead.-
Nick was more than happy about this as it meant he could slowly assimilate the knowledge within the lexicon without worrying about the mind of his body getting fried. Daedric brains were durable but if you suddenly shoved hundreds if not thousands of years worth of knowledge into it and that durability was bound to be overwhelmed. Nick wasn't worried about his own mind getting damaged as it was connected to his essence which was not entirely present in the bodies he piloted. If one of them get overloaded like that he would get a bit of backlash but would otherwise be unbothered by it.-
There was one rule that sort of limited his ability to multitask however in that he can only control one body in one realm so both of his bodies have to be in a different realm if he wants to control them both at the same time. Still that rule was not a problem at all since any sort of multitasking was better than none at all. As for the knowledge that the dwemer put into the lexicon it would be better to ask what wasn't there. Daedric anatomy , crafting techniques , geology , herbology , high end mathematics , you think of any subject and the lexicon held at least something on it.-
The part that got Nick excited though was when a chunk of knowledge regarding tonal magic popped up. Tonal magic was a fascinating subject that very few on nirn ever learned even a little of as the dwemer were highly protective of it. The idea was basically that all things had a sort of tone to them and if you could recreate that tone you could in theory recreate that thing in it's entirety.-
There was more to it than that obviously but that was the basic premise of the magic. The dwemer were especially prodigious at this form of magic as it was even the way by which Kagrenac zero summed the entire race. By messing with the tone of a gods heart and trying to fuse it with that of the tone of the dwemer race. More likely than not he was actually intending on forcefully ascending himself to divinity to fight off those attacking at the time but ended up dragging the rest of the race in the realm alongside him to nonexistence.-
It was an arrogant thing to try and steal a gods divinity to force oneself to that level and Kagrenac paid the price for it. But that was an extreme example of tonal magic when the best use of it was in another more subtle way , resonance. By creating a tonal resonance one could make nearly any piece of magic or enchantment dozens of times more powerful. It was a bit like turning up the volume on a speaker to make music louder. The music itself doesn't really change but it carries further and can be heard more clearly like it had changed.-
Nick wanted tonal magic specifically for the uses in crafting that it would have that would let him take another step forward on that path. His crafting domain was going in an entirely different direction than any other craftsman before him so it likely felt totally different than Hephaestus's own. Magical beings with similar domains felt it after a certain level of power and yet Hephaestus was entirely caught off guard by the reveal that he was a craftsman. Nick couldn't blame her since even he couldn't feel her crafting domain as they were nothing alike.-
Nicks domain was made of each of his styles and his experiences with them as well as what he has created on his own or learned from others. As a result it was full of concepts and things that weren't from this world or even the same world as each other so there was no one in the world with a similar crafting domain. What he did share with Hephaestus however was a fire domain as they both walked a similar path in that regard though with her obviously being much further ahead of him in it.