Raymarine fishfinder - sharing transducers?

Likely not. Most of the fishfinders require a higher spec transducer than depth gages. Have a check of the data sheets for the fishfinder.
Adding a separate transducer isn’t a drama
 
Doubt it. The transducer in most set ups turns an electrical signal into a sound wave (ultrasonic) then turns the reflected soundwave back into an electrical signal. The 2 units would interfere with each other.
 
I have a bidata st 40, kept the original through hull transducer, I then added a fish finder mfd, I fitted an additional transducer on the other side of the boat, an airmar p79 in hull. Both seem to work ok, I was concerned that there may be some interference but all seems good.
 
Adding a separate transducer isn’t a drama

Yes it is or was in my case!

The transducer needs to point down in both athwartships and fore and aft directions.

I fitted mine on the centreline before the depth sounder transducer but it required a substantial fairing to bring it to a vertical direction.

It depends on the hull shape of course.
 
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