That attribute is not an HTML or CSS standard. It is a costum XML attribute for OL tags. I think DocBook is a publishing system buildt in xml that can output html, and it uses that attribute.
If you don’t use javascript inserting new ol tags after the 50th item I think your best option is to float all the items in one list, but with the result of odd items and even items in each column in turn.
If the lists are not dynamical I suggest two lists contaning the item sequence.
Semantically, this doesn’t make sense. If the list sequence is interrupted, then it is a new list sequence, and can’t be a continuation of a previous list. If your contents is really an ordered list (i.e. where the order of the items is important), then you must use only one list. If you are merely using the <ol> tag to add numbers in front of the list items, then you have an unordered list and must mark it up with the <ul> tag.
As Christian says, if it’s one list it must be a single <ol>. That’s the only semantically correct solution. You can use CSS to position the items in two columns.
If it’s semantically two separate lists, but you want them to appear as a single, continuous list, you can use CSS to change the starting number of the second list and CSS counters to render the numbering.