I'm about to replace my MFD8 with a TZT9. I'll probably also pick up a DFF3D soon and would like to know how a dual sounder (DFF1 and DFF3D) system works with the TZT. I remember reading somewhere that you can have two sounders on a navnet system but need to know how it works. Can you only select one at a time? Is it simple and fast to switch between sounders? Is there any documentation that describes this in more detail?