"two wives": This tells me only of two. Whence do I derive (that the same applies) even if they are more? From "wives" (in any event).
This tells me that the double share of the bechor (the first-born) applies only if there are many wives, some loved and some hated. Whence do I derive (that the same applies) even if all are loved or all hated? From "and they bear him sons, the loved one and the hated one," (the implication being) even if all are loved or all hated.
This tells me only if there are many and all are loved or hated. Whence do I derive (that the same applies) even if there are only two? From "two wives".