It will work to a degree but because the comutation is done electronically with a brushless motor the motors would have to be spinning at exactly the same speed and the rotors at exactly the same point to get the desired effect.
This is impossible to make happen efficently and they will be working against each other at least part of the time and neither motor will be running at its full potential. Ive never tried it but Id guess he would get better performance just running a single motor which is why its not recomended.
People who say you cant do it are probably just repeating what they have heard and dont undenstand why you shouldnt do it. As a result they should be banned from ever posting any advice or information on any forum or web site to make the web a smarter place.
Nothing like a good rant of an eve ;)