In case you want to use more than two consumers try this:
Put a Content Enricher in front of the message consumer. Let the Content Enricher calculate a hash value that is something in the range from zero to one. Choose a simple and predictable hash function. For an order number you could divide the last two digits of the number by 100. Save that hash value at the message, let's say in the property X.
Then you would configure three message consumers with the following message selectors: "X < 1/3", "1/3 <= X and X < 2/3", "2/3 <= X".
If you are allowed to change the message sender, add the property before the message is sent. In this case the content enricher is redundant.