As long as each speaker has enough air space i don't think it matters. People will do two chambers as a precaution incase one speaker blows or somethign happens to it, it won't affect the other speaker. The brace will make the box stronger which is always a good thing. Vented boxes are very sensitive to slight volume changes in the box. Sealed can be off by a fair amount before it will make a noticable difference. I would suggest starting with a sealed box. Make sure you take all you measurements a few times to make sure you have them right. I suggest calking the all the joints to make sure no air will leak. Besides that there isn't much to it besides cutting and assembling. To brace it just cut out a piece of mdf the size of the interior of the box and just screw it in so each chamber is the same size (measure). I would use atleast 3/4 in mdf.