There is only one "Ext Amp" controller output on the Helix right now. Many modern and even older amps require at least a couple of "Ext Amp" controller outputs to take full advantage of the channel switching and features available on the amp.
Don't know how the "CV" output is wired, but if it is possible it would be nice if there was a way to modify the firmware settings to allow the current "CV" output to double as a second "Ext Amp 2" controller. Both external controllers should allow flexible firmware switching of stereo (TRS) plugs to accommodate as many amps as possible. I think the number of people who have a "CV" requirement will be far less than those who would like to have a second controller output for their amp.
You can vote for it here in Ideascale
I haven't built one of these... I just re-drew Doughadfield's design as a courtesy to show how it could be mounted into a small plastic box...
I suggest getting someone in your area who is proficient in electronics to build you one... (the below schematic is much clearer than any pix of an actual wired box could be.)
 1/4" stereo jack,  1/4" mono jack,  2N7000 Transistor and  SPDT switch and some hook-up wire. The wiring is soldered to the components to match the schematic and mounted in a small plastic box.
I hope this helps.
P.S. As mentioned before... if you are having hum from ground-loop problems... the above will not help. you will need to isolate using relays and an external power source as follows...