Likai

I just install two Artes-Falcon JF-80 in my C-ARF Eurosport today, when I start one engine I can read same RPM from HDT of the other engine. Because Artes uses Hall sensor for RPM pick-up, I think if two engines are too close, their sensors will be triggered by each other and shows incorrect RPM reading in HDT. In order to prove it, I place 3rd engine between them (without any wire connected) and turning it by compress air and RPM data shown in both HDT again.
How can I fix this problem? Help me!