Is there anyway to get both Thadon's Crown AND Nevershatter from Syl in SI? Please fully explain and thanks in advance.

Easily: do the entire SI main quest line. You assassinate one (and get their item) when taking their throne. The other then turns on Sheogorath, and becomes a Priest of Order- who you have to kill in one of the final stages of the quest line, thus getting their item.

Just paly the quests, whichever one you choose first, you will get the other later.

Thanks I've played it before just its been about 2 years so I couldn't remember. But anyways, thanks!