"Multiple Sound Mixes can be active at the same time, all contributing to the overall audio effect. You can Push (Activate) or Pop (Deactivate) Sound Mixes directly inside a Blueprint with the Push Sound Mix Modifier and Pop Sound Mix Modifier nodes or activate them passively whenever a sound with a given Sound Class is playing within a specified threshold.
This means that you can activate the Passive Sound Mixes via Blueprints at the same time.
"As the EQ Settings of multiple Sound Mixes cannot be combined, the EQ Priority allows you to control which active mix’s properties are applied at any given time.
This means you cannot have multiple EQ settings overlapping. You can still play the sound mixes simultaneously, but you must set the priority of your EQ settings so they can be applied correctly relative to their Sound Class.