Mike,
With what you have available, you can do a number of different things:
1. You can run a DFF1/TM258 at the same time as the DFF3D. Matter of fact, you can have them both on your TZT2 screen at the same time.
2. You can do away with the DFF1 and connect your TM258 directly up to the TZT2 transducer port. This will also allow you to run the DFF3D at the same time. This would be my recommendation because it removes the DFF1 from your network. The DFF1 is not needed since your TZT2 already has a great sounder built in.
To make sure we are clear though, the one thing you cannot do is transmit a DFF1-UHD and DFF3D at the same time because as the DFF1-UHD "chirps" across the frequency band, it will interfere with the 165kHz of the DFF3D. You can have them both on the same network, you just have to ensure while one is in transmit the other is in STBY. This is the setup I am running on my boat and it works great. From time to time I do forget to put one or the other in STBY and you realize it right away when you see the noise on the sounder screen. When it happens, I just put one of the two units in STBY and the problem is solved.
Stickman