Connecting Force Low to ground on rear terminal

I'm using two SMUs on a 4-terminal DUT.  I'd like to have the two SMUs, which are both sourcing voltage and measuring current, to have their Force Lows be earth ground. When I connect a lug cable to the chassis in the back, I see that I'm connecting earth ground to the shield of the force low terminal, but not the pin/inner conductor of the force low terminal. 

My goal is to have those force low inner conductors both sinking into ground. How do I go about that?

Thank you. 

Sounds like you have a model 2450 SourceMeter.
You could use a triax shorting plug on the Force LO to connect the center pin to outer shell which is at chassis ground.
Keithley part number for shorting plug:  CS-1546
Info about CS-1546 Triax Shorting Plug

Thank you Andrea for your response. You are correct I am using two 2450 SourceMeters. I've submitted a request for a quote. 

Is the shorting lug the typical method for such a scenario? I'm surprised that more folks don't need a common low between multiple SMUs.