There’s a setting in CSS called counter-reset, but it doesn’t seem possible to do what you want using it. There are browser-specific multi-column layout tricks you could try, but really, this isn’t going to be possible using a real list.
You could try faking it with a table (a separate column for the marker numbers and the text, and a separate row for each item), but you should probably give up and make your numbers some other way than using an ordered list. It’s a shame, really, since semantically this is precisely what you want to have, but the format doesn’t let you have the layout you want.