Storing onions and potatoes in the same cellar
I believe that advice is mainly meant for people storing a large quantity of potatoes and onions (like two 10lbs bags) for long periods of time (like 2-3 months).  I can't imagine it applies to a few potatoes and onions stored in a basket.

I have a pantry which is consistently between 60F and 68F, with a humidity of 60% to 80%.  I have stored potatoes and onions together in a hanging wire basket for up to 4 weeks at a time. While the potatoes will turn green and sprout in that time (I buy organic) the onions are still good (although somewhat stronger after a month).

So my answer is: ignore that advice.