I've seen a lot of stuff out in the blogosphere about neon colours and layering them over white. I decided to test this method myself, using Australis Totes Amaze, a neon pink. I used Illamasqua Scorch as my white base.
You can see what layerings I did in the photos. I found that the nail with only the pink had more of a visible nail line (it's not very noticeable in the photos, but it was there in person) and was also a tad less bright. The neon over one and two coats of white looked the same.
If you want to get rid of any visible nail line and ensure brightness of neons, I'd go for one coat of neon over one coat of white; but, of course, you might have to use different combinations depending on whether your neon polish is streaky or more sheer.