Can I turn my 2-way Bookshelf speakers 90°…?

In the time it took you to post this, you could have turned them on their side and found out. 

I have faith that you can do it

It works OK for some of mine, at least for rears.

You want the tweeters to disperse at seated ear level. Horizontal can be great for that

Typically you want the tweeters on the inside. However, in my case, they sound better with the tweeters to the outside, because the left speaker is near a side wall.