Assuming the KDF is actually secure,There is no clear advantage other than domain separation, and certainly some severe disadvantages (code complexity and size).
If you have the ability to change things, I would simply use one call to HKDF-SHA256 or HKDF-SHA512 once to get all the bytes you need. SHA-256 is hardware accelerated on some platforms so you might prefer that.
Many crypto libraries have HKDF, if not it is trivial to implement correctly if you have an HMAC primitive available.
UPDATE: I now see that the latest ANSI X9.63 still specifies SHA-1 as its hashing primitive. SHA-1 is broken in terms of collision-resistance. While that doesn’t immediately affect a KDF (which relies on pre-image resistance), it does means you should clearly replace it with something stronger as soon as possible. SHA2-256, SHA2-512, SHA-3, BLAKE2 are all reasonable choices.