The phrase collaborate together is a redundancy. The word collaborate means to work together or to cooperate, so the concept of togetherness is already implied and does not need to be reiterated.
Examples
In these examples, the word together could be removed without loss of meaning:
The two were trying to collaborate together on new material Jackson was preparing. [MTV]
. . . but only the companies with an advanced outlook on how they allow their staff to collaborate together are set to gain the most advantage from this shift. [Telegraph]
They have started using blogs and wikis to collaborate together on difficult, unsolved mathematical problems. [transcribed in Boston Review]
If you feel the word together is necessary for what you want to say, consider using the nonredundant work together.
