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

My Roksan Darius MkIV (from the glorious 90’s) were designed to work like that, in a 90 degree angle. In fact I am listening to them right now, and the soundstage is huge with pin point imaging. Although very tough to set-up, all efforts were rewarded with sublime sound: they completely vanished in the room, and deliver very tight bass, clear and sweet treble.

That said, I never experienced that extreme toe-in with any other speakers, in fact, I prefere no toe-in at all with my Thiel, Totem, Kef and Quad speakers.

It's not ideal for imaging, and the phase relationship with the woofer at the crossover becomes an issue.  But if you don't care about that then go ahead.

Properly engineered 2-ways generally exhibit wide, well balanced response in the horizontal plane, but often some noticeable irregularities in the vertical plane, due to the ways frequencies shared by both drivers tend to interact in the crossover band.  For a vertically aligned speaker, the tweeter height is not the real issue, it’s finding the sweet spot where the midrange is not “sucked out” by phase cancellation, and a combo of height and toe-in is usually successful.  Rotating to horizontal makes the likelihood of achieving this result far less.  I suggest using “coincident” 2-ways if you must do this.  KEF, Tannoy, and Elac are known for this and more recently MoFi.  Andrew Jones is a big name in this field.

I have a small set-up in my hobby workshop, a 10' x 12' room that has a 9' ceiling. It's occupied by two work benches set in one corner to form an "L" with wall mounted cabinets over them and one seated work station per bench. Since the opposite corner holds another work bench, the only available floor space holds an 8" sub in one corner. My two Elac B6 Debut's are sitting upright on top the wall cabinets at each end of the "L" at 90° to each other. This works surprisingly well, and with a tweak of the balance, the 'sweet spot' becomes which ever seat I'm sitting in. It is possible to make the system work with the room when the room dictates how the system can be set up.

Appreciate folks for helping me understand this dynamic.  Am still learning....and learning I have more to learn!!

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

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/

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.

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.

I have faith that you can do it

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

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