Does two center channels = one stereo system

Hypothetically speaking yes, you have stereo albeit not a very good one as Rar1 pointed out.

frequency aside , what about the quality of the sound
sound stage, midrange..etc

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.

Rich

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.