Humminbird + lowerance = interference

I bought a humminbird 998c about a year and a half ago, after adjusting the transducer several times, I gave up on ever seeing a good reading of any sort over 10 mph. I heard about a high speed transducer that could be used for down scan and regular sonar, but was told by several "experts" that it would not be a good option for me because:
1. I have an aluminium deep vee and the high speed transducer wouldn't work for installation reasons? It is apparently only made for fiberglass boats.
2. Because the top speed of my boat is 35 and I cruise at 25 or so my boat was to slow for it to be effective.
3. The high speed transducers are unreliable and not worth the cost.

I had an elite 5 dsi and was very impressed with the downscan feature, which I still favor over the 998's. One thing leads to the next and I decide to install the new elite 4x along with the 998, both on the transom. This leads to a pretty decent amount of interference in both readouts. Is there a way to avoid this without turning a transducer off? Are the "experts" crazy, would I be better off with the high speed transducer for the humminbird?

The HB 998c HD SI transmits the 200kHz frequency all the time...if there is a setting in the Elite 4 that allows a different frequency choice other than 200kHz...there should be no crosstalk interference...
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High speed readings with the HDSI xducer are sometimes difficult because of the size and dynamics of the HDSI xducer housing shape...this is not to say that it is not possible to get high speed readings with the HDSI xducer...

Proper xducer placement is paramount in that the xducer must have constant bubble free water across the face of the xducer...any air whatsoever passing across the face will cause the xducer to "lose" bottom and not read properly...

If your application does not allow lowering the HDSI xducer enough on the transom to accomplish constant bubble free water flow for the HDSI ...then an additional Dual Beam xducer along with the AS SIDB Y cable will allow the HDSI xducer to be mounted a little higher on the transom for idle speed SI/DI imaging....and allow the additional Dual Beam xducer to be mounted lower on the transom to obtain high speed 2d depth readings...

The XNT 9 20 Dual Beam transom mount 2d xducer is much more conducive for high speed reading because the housing shape allows easier adjusting to get constant bubble free water flow for continuous high speed 2d readings...

While most discussions of an additional Dual Beam xducer are for applications is glass boats with the use of a "Shoot thru" Dual Beam xducer...the transom mount XNT 9 20 can achieve the same results for metal boats...

Rickie