It is possible for two authors to own an interest in the copyright of a work if both authors contribute to the creation of the work. The contribution, however, must be relatively substantial; merely suggesting ideas or titles does not give rise to a claim for joint authorship. There must be a contribution of actual work.
If you collaborate with someone, it is essential to have a collaboration agreement to govern the work. In such an agreement, you can specify which authors receive what percentage; if it comes down to determining the interests in a court case, the split may be fifty-fifty. If you don't want this to happen, it's a very good idea to enter into a collaboration agreement right from the start. This will clearly specify each author's respective copyright interests, while also protecting your individual legal interests with things such as payment, expected contributions, and so forth. There are a slew of sample collaboration agreements available on the Internet. You can use any form you like, as long as it addresses the following five points:
The parties to the agreement, with full contact information clearly spelled out
The expected contribution of each of the parties
The agreed ownership split
Provisions for the division of advances and royalties
A clause that stipulates the name of the literary agent to be used, if applicable