Beyond TV with 2 x Motorola DCT700 problem (zones)

I have a setup that works properly with a single DCT700 and the USBUIRT. But I want/need a second DCT700, and so have made a cable that connects the tip and sleeve of a cable from a 3.5mm stereo plug to an IR emitter. I have tested the cable with an ohmmeter and am satisfied that there are no shorts.
But.
When I send a channel change command to the Zone 2 emitter, it flashes and changes the channel. On both boxes (both the cable emitter and the built-in USB-UIRT emitter flash) with every command.
I have shielded the Zone 2 emitter from the other box and vice versa.
What am I missing?
But.
When I send a channel change command to the Zone 2 emitter, it flashes and changes the channel. On both boxes (both the cable emitter and the built-in USB-UIRT emitter flash) with every command.
I have shielded the Zone 2 emitter from the other box and vice versa.
What am I missing?