good point. FCMP++ will be an upgrade to the Monero protocol through a network upgrade. to realize the privacy benefits of it, you need a new addressing scheme. so far the scheme was planned to be Jamtis-RCT. Carrot is a separate addressing scheme to be used with FCMP++. nothing prevents the simultaneous use of both, and they look the same to outside observers. FCMP++ was basically "upgraded" by adding another potential addressing scheme to it.

this doesn't guarantee that both will be used. my guess would be that the industry will converge on one addressing scheme.