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

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.

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

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.