On the basis of the distribution of the polymers between the separating phases, the phase equilibria of mixtures of two polymers in a common solvent may be divided into two main categories : In an associative phase separation, both polymers are enriched in one of the separating phases, and in a segregative phase separation, the two polymers are enriched in separate phases. This work investigates the conditions for the two types of phase separation, as predicted by the Flory-Huggins theory. The emphasis is on the (previously largely neglected) associative phase separation and on transitions from segregative to associative phase behavior (S-A transitions) through changes in the pair interaction parameters. An associative phase separation is always favored by marginal or poor solvent conditions and may sometimes occur even without an effective polymer-polymer attraction. An S-A transition may be generated by changing the polymer-polymer interaction but also, under certain circumstances, by changing only one of the polymer-solvent interaction parameters. When one of the polymers is only partly miscible with the solvent, a continuous S-A transition may occur, where the same two-phase area gradually evolves from being segregative to being associative. A continuous S-A transition always occurs via a region of borderline phase separation, where one of the polymers distributes equally between the two separating phases. The borderline conditions are also conditions for maximum miscibility (a minimum extension of the two-phase area) in the system.