Not quite. If you look at the frequency ranges for center channel speakers, they tend to bottom out in the 70-80hz range. So, unless you plan to use a sub-woofer, you will be missing out on the bottom end.
In theory, 2 centres should work as well as regular main speakers. Of course sound quality will depend on the speakers. There are a number of regular speakers using a centre positioned tweeter (Linn Ninka, JML utopia mini come to mind), so that shouldn't be an issue. And, depending on the model, it is usually voiced to match the regular mains (ie., sound the same).
But, as always, you have to listen for yourself and decide.
Aside from the missing frequencies, there is no reason why the sound can not be good. I have often thought about going this route as I have speakers placed inside bookcases and I often must turn the speakers on their sides, as I do not have the bookshelf height available to allow the speakers to stand up. So, since my set-up is so compromised, would using two center channels be all that worse. Don't know for certain.
Where I have made the compromise is that I would rather have sound with some lower bass presence than have niceties like soundstage. I didn't think that center channels could do the disappearing act like well positioned monitors can, but I could be wrong.
Two center channels work best when they are the same make and model as the left and right. In fact the best center channel is always obtained by using the same speaker as the left and right. Of course this is only true if you can align the drivers of all the speaker in the same plane. Most center channel speakers are the way they are to accommodate placement and size, not better sound.
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 ).