Typically when a song is licensed into a TV or Film project there is only one publisher. If the writer of the song hasn't assigned his or her publishing rights to a third party, they are by default the publisher of the song. In other words, if you write a song, you are the songs' publisher and are entitled to all publishing rights unless you give them to someone else.
Having a good publisher on your side can be a valuable asset if they have solid relationships with music supervisors. In fact I typically suggest songwriters seek out publishers and/or licensing agents and the like when they are first starting out, because in most cases it's easier to get music placed this way than functioning as your own publisher and trying to cultivate relationships with music supervisors yourself. There are always exceptions and every case is different, but as a general rule I suggest songwriters seek out third parties to help them pitch their music and this almost always entails giving up some or all of your publishing royalties for songs that are successfully placed on your behalf.
Yet another variation on the typical songwriter/publisher relationship is something known as "co-publishing". The way this type of deal works is really simple. Under a co-publishing arrangement, a third party will shop your music, but instead of taking all of the songs' publishing for music they successfully license, they only take a percentage, usually 50%. This leaves the remaining 50% of the publishing as well as 100% of the writer's royalty. The remaining 50% of the publishing could go directly to you, if you as the writer initiated this type of deal, or someone else if they've initiated this type of deal on your behalf.
This is the route I suggest new publishers and licensing agents take in the beginning of their ventures. New publishers probably don't have the neccessary relationships in place to start licensing music. Co-publishing deals offer an alternative to traditional licensing deals. The benefit to the more established publisher is that someone else is bringing them music they probably wouldn't have known about otherwise, and the benefit to the lesser established publishsher is that they are able to benefit from the connections of those they enter into co-publishing arrangements with.