Hi,
This can be done in several ways. One easy one i can think of right now:
You can scale and size each mix window so that it becomes one part of the total. This can be done by using the size-offset-positon parameters of each submix... See attached screenshot. This example has 4 submixes but offcourse you can do this with as much mixes as you would like.
Need more info?? Contact me directly.