As far as I know it is customary, but not necessary to have two, because in the case of an adult convert, it can be that only one person fulfills that role, and is called a "sponsor" -- and being either male or female is not an issue. It's not a gender based role.
What the issue, as far as I know, is that the church at this time considers a person "naturally" or according to "God's will" to be of the gender they were born to. So that, when a person takes actions to change that about themselves, rather than "reconcile themselves" with what the church considers to be the will of God, that is a matter of not living according to the church's beliefs and/or teachings. Therefore, that would disqualify a person as being one among many issues, that might -- due to the person not living a Catholic life -- and therefore, not qualifying for a role that is reserved only for people that actually believe and are living according to Catholic teachings in their own life.