center speakers in stereo

wondering if anybody has tried using 2 center channel speakers as mains? I've got a monitor audio silver c350 & was thinking of adding another for 2 channel use in a spare set-up.  Are the crossovers perhaps engineered so they wouldn't work well for this use?
You certainly can do so. Most passive center channels may be limited in their frequency response, but are suitable for such application. 

Looking at that particular speaker it would make for some interesting variations if you wish. Since the tweeter is offset, you could conceivably use them in four different placements. You could experiment with what I call Landscape Orientation and Portrait Orientation. See my system pics for a larger setup or two done in Landscape. It's quite lovely alternative to the traditional Portrait orientation. 

If I had lower budget and liked the sound of the speaker I would very possibly do precisely what you are thinking. That is not to say there might be some speaker out there to outperform; there are plenty. But, for your purposes and with the variety of tweeters inside/outside in Portrait, or Tweeters up/down in Landscape - that's a lot of potentially good outcome. :) 

Do not forget also that baffle slope is important to fix a speaker's performance more sharply. If the speakers sit low, place a small shim under them to lift the baffle/face of the speaker such that the mid/tweet are aiming at your head. It improves the sound quality nicely.  

(I am not interested in debating anyone about my advice or methods.)
Set up horizontally they should be fine. Vertically however you are going to have to carefully consider where to point the tweet. Probably on the outsides, and towards the outer edge of your listening area.

I have often wondered about that .I also thought you could or 3 of the same .in home theater. 
I think it's hard to set up a great multichannel system without using a processor. Possible, yes. Optimized, no. Some of the best "stereo" imaging I ever heard was playing through a modest Meridian system using their proprietary 3-channel processing. Would have done iot at home except that I didn't like the proprietary nature of the Meridian system.