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

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.

The answer to this has been covered a few times lately. For a typical 2-way bookshelf speaker, based on crossover limitations, you want to listen at the tweeter level or below. Above the tweeter response can be weird.

So if you turn the speakers to their sides you want to position so that the listener is in the same relative location. Another way to put it, you want your ears to be between the tweeter and mid-woofer axis.  Sometimes even on the mid-woofer axis, so arrange the speakers so your location tends to stay in this range.

If you review several Stereophile reviews and take a look at the off-axis plots you’ll see why this is.

Yes you can without issue BUT the sweet spot will be pretty small if they normally have vertical drivers.  At this website you can see horizontal vs vertical dispersion.
 

   https://www.erinsaudiocorner.com/loudspeakers/sonus_faber_lumina_ii/

PS-  The limitation is with low-order crossovers.  4th order filters, rarely used in passive 2-ways, avoid this problem pretty much.