Maybe a recap from the top? Two speakers are two speakers. Plug them into the right bits, they make a stereo pair. Full stop.
Plug any speaker into a center channel output, it becomes a center channel speaker. In short, all speakers are all 100% interchangeable.
Now, the real question seems to be, how would a speaker designed (and perhaps more apparent, albeit less determinative, marketed) as a center channel speaker sound as a stereo pair. That question, of course, depends entirely on the speaker. Some center channel speakers are specifically designed to be used in either application either a stereo bookshelf speaker, a single center, or all three (Thiel, with their coaxial drivers, comes to mind, as thats what I use). Some other makes might, for whatever reason, work less well.
So, generally and hypothetically you bet you can, speakers is speakers. Beyond that, are one set of bookshelf speakers going to sound better than another due to more factors that we could possible list here? Again, sure are. Do all bookshelf-sized speakers share, to one degree or another, certain inherent design limitations? Yup. But if you like the way they sound, youre done (until, that is, you find something better and rest assured, there will always, no matter what, be something better
).