There will most likely be crosstalk interference because HB units transit 200kHz all the time for digital depth ...
Each Setup is different and how much crosstalk interference you see with both units transmitting cannot be known without mounting and testing on the water ...
There is a "Noise Filter" setting under the Sonar tab that can help with interference ... but how much interference this setting can filter out is also specific to each install ...
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If the HELIX is also a SI unit - it is a challenge to mount both xducers on the transom without physical blockage of the SI pulse from the other SI xducer (the SI beams reach all the way to the water's surface on both sides of the xducer) ...
Mounting SI xducers on opposite sides of transom can minimize physical blockage of the SI Beam somewhat ...
If the transom is stepped - a better option for 2 SI xducers is to mount 1 on the lower step and 1 on the upper step to effectively have 1 SI xducer "in front of" the other ...
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The 899 and HELIX will also need the AS EC QDE Ethernet adapter cable ...
http://www.humminbird.com/Products/AS-EC-QDE/Rickie