What happens when two players both play Control Magic on the same creature?
The last effect created takes precedence.
Section 613 resolves interaction of Continuous Effects. The rules relevant to this specific question are the following:

613.6. Within a layer or sublayer, determining which order effects are applied in is usually done using a timestamp system. An effect with an earlier timestamp is applied before an effect with a later timestamp.
613.6a A continuous effect generated by a static ability has the same timestamp as the object the static ability is on, or the timestamp of the effect that created the ability, whichever is later.

Continuous effects are continually reevaluated. This means that destroying the second Control Magic gives control of Pathraizer to the controller of the first Control Magic.