My nine month old is only feeding from one side- and has been for about 6 weeks. I can get him to feed from the right side only if i hold him as if he was feeding from the left and sort of twist - even then he is not keen. There definitely doesn't seem to be as much milk from this side (it is almost impossible to express) - but it is still producing!
I think I started this by favouring that side but am not sure why he keeps refusing - he seems to be getting enough from one side only but I am not sure if it could cause other problems - or whether I'd just get really lopsided!