A proportion of our onions, last year and the year before, bifurcated, making them a real pain to prepare for cooking! We have very good soil which we feed with seaweed, compost and well rotted horse manure. No-one else in our local GIY group had experienced the problem or could suggest a cause. The red ones were not affected. We used different varieties each year – turbo, sturon and stuttgarter reisling (? spelling?).
This can happen sometimes with certain batches of onion sets, usually large sets that might have divided within the scales and this cannot be seen on the outside. Sometimes you can make out the double shape and you can see two shoots emerge from the set at an early stage.
You could check the sets by pulling away the outer scales and taking off one part of the double if you wished. Occasionally, very strong early growth could cause a bud to split but usually this double bulb is present at planting.