I have a Helix 10 DI at the bow and a Helix 10 SI at the console. The DI transducer is the TM transducer and is mounted on the Ultrex while the SI transducer is mounted on the transom. Since everything is networked I can see the data from any transducer on any screen. I also have my console unit on the Precision Sonar mount (awesome very solid mount BTW). I used the 15' Ethernet cable and have the Ethernet switch located inside the console. RenegadeBassin also used the 15' cable (he said so on page 4 of this thread).
If you have everything networked you certainly don't need an SI unit at the bow if you already have one at the console since you'll be able to see the SI data from the transom SI transducer on the bow unit. It would make more sense to have a DI unit at the bow with a DI transducer mounted on the Ultrex since this will give you DI data directly beneath the TM. If I was you I would put the 10 SI in the console on the Precision mount and ram mount the 7 to the side at the console. Then get another 10 DI for the bow.
Alternatively, you could flush mount the 7 in the console and ram mount the 10 to the side of the console like Renegade did. This would eliminate the need for the Precision mount. You will still need another unit for the bow though.
Good luck!