12
Given that the rule is that you want every second string in one list and the others in another list, you can do something like this:
string s = "AAAA,12,BBBB,34,CCCCC,56";
var parts = s.Split(',');
var first = parts.Where((p, i) => i % 2 == 0);
var second = parts.Where((p, i) => i % 2 == 1);