I have been growing these onions for many years with pretty good success. However, after last year getting more doubles/splits than ever, I did some research. I have always planted five inches apart. I found that the experts recommend four inches to prevent splitting. I tried that spacing this season and can report that so far I have about 0.5% splitting in my patch (one plant out of about 200). Amazing! Live and learn. I love the internet!
The biggest problem with splits is they rot faster.