If you have more than 1 xmtr that you want to use, it is best to do your mixing in G3 rather than in your xmtr. You can accomplish this in the aircraft editor in the "Software Radio" section. Here are some generic mixing instructions. You can substitute any number 1-9 wherever you see 'N'.

Right click on Output Channel N, and choose to Add Complex Input Feed. This will give you a mixing curve to alter Channel N with.
Select the new Input. It will be the last Input in the Output Channel N tree.
Change the Input Channel to Input Channel N
Click <<Graph Points>> next to the Curve item, then click the little [...] button that appears on the right. This will allow you to edit the mixing curve.

Optional Steps, for some cases:
Expand the Input Item and select the Input Feed When item.
Change the Activated when value to Conditional Channel above/below value #1
change the Conditional Channel to Input Channel N
Set the curve points and click OK.