Recently I have done a number of readings for people who are already made in Ifa, Sanse or other traditions. The question of "mixing and matching" traditions frequently rises in ATR circles. Generally it's frowned upon, with good reason. Most of the Voodoo Celtic Shaman Buddhist Santeros out there have little or none of the training which should come with the titles they claim. (Not to mention the folly of assuming that "African religions" are all substantially identical or that there is such a thing as an "African culture." Nobody assumes that there is some overarching "European religion" which stretches from Scandinavia to Sicily).
That being said, there are a number of reasons why somebody who is initiated in one African tradition may be drawn to another. One Mambo in our house is also a Palera: since her father was Haitian, her ancestral spirits insisted that she kanzo. And there are many practitioners of Ifa and Las Reglas de Ocha who are also scratched in Palo (Las Relgas de Kongo) -- although quite a few Paleros bristle at the cavalier treatment accorded their tradition by people who are primarily interested in the Yoruba tradition.
You don't have to become an initiate to learn from another tradition. Most Vodou ceremonies are open to the public. Our house has regularly welcomed Babalaos, Espiritistos and other practitioners to our events. Discussing the similarities and differences in our practices has been mutually beneficial and educational. We haven't tried to convert each other, nor have we tried to play the "my tradition is better than yours" game. Conversely, we haven't tried to "pull rank" and claim that we were experts in Ifa etc. because we are Houngans and Mambos, or vice versa.
One thing I've noticed about initiates (and serious spirit-workers in general): they are reluctant to take on new commitments. Newcomers enthusiastically plan to become high-ranking members of a dozen different traditions. Those who have actually achieved some degree of attainment know that mastering one path requires enormous effort. Being told "you are called to be a priest/ess" means you are called to spend a lot of money and time on training, then undergo arduous ceremonies and take on new taboos and responsibilities after your initiation. It's not just a matter of getting more hit points and access to higher-level spells: it's a lifelong commitment. The initiates I've read for have generally expressed relief that they weren't being called to kanzo. They have quite enough on their plate as it is.