i had the same set-up
changed to ST 60s one transducer on the centre line works fine ( now use the alarm /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif) on my fin keel flatish bottomed boat. transducer fitted inhull forward of the keel by approx 2 ft
I went to change my Raymarine ST30 display for a ST40(on previous boat).People on Raymarine stand at LBS said OK to use transducers just swap heads.However I E mailed Raymarine & they said the ST40 log would not work as it has a temperature reading as well as distance/speed etc.As the original tranducer does not have this facility the instrument thinks the log does not work & displays zero.It can be overcome by soldering a resistor in place.This all got too technical for my simple mind & I gave up.Info available on Raymarine website.