How do I split revenue from IP with different owners?
It's actually the same answer regardless of what type of IP you're looking at - you have a contract and you follow what the contract says. Now, how do you come up with the system the contact spells out? Negotiation. Long, hopefully pleasant, negotiation.
When do I need to apply for one?
In the United States, trademark rights arise from using the mark with goods or services in interstate/foreign commerce. You can apply for a trademark when you think of the mark, but it will not be registered until you can show you have been using it. Or you can delay applying to register the mark, and you can register it later when you determine the mark is worthwhile.
In general, it may be valuable to register a trademark only when you are starting to market a product or service with that mark. It can make sense to wait until the product and its mark are relatively popular, or it can make sense to register early if you expect more than one product to be associated with the mark (such as if you are using your company name as the mark). Early registration is better for legal protection, but you likely do not want to waste money on marks you are not sure you will be using later.