What is the difference between a multiplexed bus and a multi-master bus
Any communications channel that is multiplex means it can be accessed by more than one transmitter.  That's pretty general; in your context it means that different transmitters can use it at different times.

A bus master is the piece of hardware that actually controls who has access to the bus.  So a multi-master bus is one that has a mechanism for multiple masters to take over the bus at different times.  Following through on this thought, a multi-master bus has some sort of arbitration scheme so that if more than one master wants control over the bus at one time, only one of them actually gets that control.