Selecting transducers these days can be dizzying.
Below are the specs for the sonar of the 8616xsv that you said you were getting.
I am not certain of the sonar connections on those units. My guess is there is one transducer connector and it can also connect to black box sonars thru the network.
It can't run 2 transducers?
Dual-frequency (50/200 kHz) sonar capable
Dual-beam (77/200 kHz) sonar capable
Frequencies supported
Traditional: 50/200, 77/200, 83/200
Single Channel CHIRP: 40-250
Dual Channel CHIRP: 40-250
SideVü/ClearVü: CHIRP 260/455/800
Ultra High-Definition ClearVü: 0.8 MHz (800 kHz), CHIRP range: 760-880 kHz
Ultra High-Definition SideVü: 1.2 MHz (1,200 kHz), CHIRP range: 1,060-1,170 kHz
(dependent on transducer)
Transmit power 1 kW traditional; 1 kW CHIRP
You need a transducer that will do the things you want..at the highest power...
You absolutely need one of the GT models to utilize the SideVu.
GT34UHD is for ultraHighDef shallow water...don't think you want that
GT41
Perfect for the coastal and offshore fisherman who want traditional sonar, CHIRP ClearVü and CHIRP SideVü scanning sonar in a stainless steel stem
The CHIRP ClearVü/ SideVü elements have a power rating of 500 W per element (1,500 W total) and operating frequencies of 260/455 kHz.
The traditional sonar has a power rating of 600 W and operating frequencies of 50/200 kHz. It includes a built-in fast-response temperature sensor.
GT50
Perfect for the shallow water fisherman who want CHIRP traditional sonar, CHIRP ClearVü and CHIRP SideVü scanning sonar,
The CHIRP ClearVü/SideVü elements have a power rating of 500 W per element (1,500 W total) and operating frequencies of 455/800 kHz.
The traditional CHIRP sonar has a power rating of 300 W and operating frequencies of mid-band CHIRP 80-160 kHz. It also includes a built-in fast response temperature sensor.
GT51
Perfect for the coastal and offshore fisherman who want CHIRP traditional sonar, CHIRP ClearVü and CHIRP SideVü scanning sonar with a stainless steel stem,
The CHIRP ClearVü/ SideVü elements have a power rating of 500 W per element and operating frequencies of 260/455 kHz.
The traditional CHIRP sonar has a power rating of 600 W and operating frequencies of mid-band CHIRP 80-160 kHz. It includes a built-in fast-response temperature sensor.
So if you are getting two 8616xsv with sonar units then you wouldn't want two GT51s.
I would opt for an Airmar B164 (traditional) . They are high power 50/200khz and will get you deep and work well while running.
You'll be able to use a range of your sonar that the GT51doesn't support.
They are flush thru-hulls, so placement is not as much an issue
Give this a read...
Airmar transducer guide