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

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.

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.