The thing about working with preschoolers is you are always having to either talk over lots of little voices or find creative ways to capture their attention. Lots of little voices all going at once is to be expected but lots of teacher voices all going at once should not be part of the plan.
Too many teacher voices
If there are two or more teachers in the classroom and each are talking equally loud to get the attention of the entire class - the result is often more chaotic then helpful. The children need to know who to focus on and for a few moments - there should be only one voice.
Not so helpful
I have observed many classrooms where one teacher is in the front speaking to the children and the other teacher will call out across the room telling the children to listen. Each time one teacher raises the volume of his or her voice to help the other teacher it creates a conflict as to who the children should look at and listen to at that specific moment.
Who is in charge?
The lead teacher would normally be the main voice in the classroom. While the lead teacher is gaining the attention of students, all other teachers should use softer voices or even body language to support the lead teacher's efforts. If a student continues to be disruptive - the most helpful thing the assistant teacher can do at that moment is to discretely walk over and quietly address that child individually by reminding or encouraging the child to listen to what the lead teacher has to say.
Ultimately, every teacher should have a voice in the classroom but the goal is to use each voice effectively so that the children are not confused by who is in charge at a given moment. When one teacher is in the lead - then let that voice be the dominant voice.
The next time you are in the classroom and you see another teacher ask for the children's attention, read a story, sing a song, or call for clean-up time; consider being the supportive voice that works behind the scenes helping to discretely guide children towards the main voice at that moment.
Remember...more than one dominating teacher voice along with lots of little voices does not lead to effective classroom management and communication.
Ideally, every teacher in the classroom needs to work together to create ONE focal point and ONE voice.