Order doesn't matter, ratios do.
The ratio doesn't have to be perfect. There is some margin of error for cases where it doesn't split right, but you generally can't do 50/50 or do 1/x-1 (1 of one type, x being the remainder) and get the proper effect.
Specific rules are in
viewtopic.php?f=42&t=41353. You might have to read through a few pages as there's a lot of examples of what people have done or can't figure out where it went wrong.
Opinions expressed in this statement are the authors alone and in no way reflect on the game development values of the actual developers.