I'm having the same problem, I don't see the fare split option. I can pick the number of people at checkout, but then I get prompted to enter credit card details and there's no option other than confirm & pay. I'm using airbnb.com from Romania. Thanks!
I successfully used this option in the past and now I have the same issue that you describe: no splitting choice when arriving at the "pay and confirm". I tried with both "personal" and "buisiness" option, same result.
Anyone has any hint on this? Does this depend on any option that the owner should validate?