What someone can do, and what someone can do legally are two separate issues, And what someone can do if someone else does something illegal is usually the issue that matters in practice.
The simple way to keep control of the collaborative product is to enter into a contract that specifies how the work is to be controlled. If your contract says that neither of the joint authors can authorize use of the song without the consent of the other, then that will generally take control over the basic doctrine. If it says that only one author can license the song, then that could work as well.
If your work on the song is truly a joint authorship, then lacking some contractual issue to the contrary, the joint ownership of the full work is a matter of law. My understanding is that the ownership so established is an undivided right, meaning that each contributor owns his portion of any part of the song, not just the part he contributed. So if the poem author wants to use his poem in another song or publication he would have to share any revenue with the melody author, and vice versa. The problem is that you may need to prove that the song was created with the intent by all the authors that it should exist as a unitary whole. That might not be the case, for example, if the poet were to consult the composer and say he would like to have his existing written poem (already copyrighted even without registration) to music to create a song. In that case the poet might possibly retain his individual rights for the poem for any other use except the exploitation of the song--the situation could depend on whether the poem was originally intended to be a lyric set to music or an independent work. A written agreement could help to clarify what other uses he could exploit, for example you could agree that he could use his lyrics on a T-shirt or publish them in a poetry collection but not set them to music to make another song.
The problem with proving what the intent of the authors was in the creation is that it is likely to be expensive and involve the courts. It may result in a lying contest, or the authors may only discover that they had a different intent from each other when a dispute arises. A well written contract can avoid a lot of problems, but it cannot cure a dispute if one of the parties is trying to cheat the other by violating contract terms, or has a basic misunderstanding of what the terms mean. In that case, it can only be a guide to a judgement, and a judgement may easily cost more than can be expected as income from the song or loss from the contract breach. Unless you trust a collaborator to be fair, and you are convinced that you both have the same understanding of the deal, you are probably asking for trouble.