Planting Potatoes with Onions?

I was planning to create mixed rows of potatoes and onions in my garden, but I've heard that onions can stunt the growth of potato tubers. Does anyone know if this is true, and, if so, what causes it?

Onions really hate competition so I think you would end up with just potatoes and some very weedy looking onions, I have never heard of onions stunting potatoes and I certainly have no problems growing potatoes where onions were the year before, I have never tried to mix them up as potatoes are such large plants and need space for hilling/mulching.

This year I planted carrots, onions and radishes together. So far its working great. add a bit of sand to thin them out.  As the radishes have come on , I pull them, and it makes space for the others. Due to my space limitations, its been a win so far.

If they did grow together, i dont see how it could be done succcessfully as potatoes need constant hilling of dirt.