[Elecraft] K3 Hum on mic between 2 rigs

[Dan Atchison via Elecraft]

I built a nice boom microphone switch to swap my Heil PR781 between my 
K3S and my K3.  The switchbox uses XLR connectors so I can use the Heil 
K3 "proper" cables to each rig.  I bought a regular XLR extension cable 
for the mic to swithbox.

Well, I used a DPDT switch to switch both pins 1 and 2.  Hum. Then I 
used a 3PDT switch for switching all pins, 1, 2, 3.  Hum.

Notice both Heil cables have outer shell at rig tied to mic low. Don't 
wanna cut this wire, or should I?

Now all this said, I am powering each rig from a separate power supply.  
My question is, before I go to the expense of purchasing a power supply 
large enough to power box rigs at the same time, will this eliminate my 
hum or ground loop?

Thanks in advance.

73,
Dan