The manual of neither device is helpful and general googling turned up nothing.
 
I have a microphone connected to a Behringer input and speaking into it lights the signal LED.
I have the Behringer set to Slave/ADAT in, and the Behringer's ADAT in connected to the Tascam's Optical Out.
The "Locked" LED on the Behringer is lit.
 
The Tascam has a 3 position pushbutton switch on the front, "Mic Pre", "Audio I/F" and "Mixer".
Neither position gives me anything on either headphone out on the Tascam.
 
The optical cable is properly seated. I have 5 cables of different manufacture.
They are all around 1/2 to 1 foot long.
I also tried changing the connections over and setting the Behringer to Master, trying both 44. and 48k.
 
And finally I connected both in-to-out and the other pair in-to-out at the same time and tried all combinations again.
 
I just don't get the signal from the Behringer coming out of the headphone sockets of the Tascam.
 
I had a Behringer interface some months ago and this part of proceedings worked without a problem.
 
Any ideas?
 
Edit: The Tascam US-20x20 works stand-alone in the Mic Pre mode.
The Behringer has no headphone output.

If you want to record from the Behringer ADA8200:
You need to route its optical output (lightpipe) to the Tascam's optical input.
Otherwise, there's no signal getting into the Tascam (from the Behringer).
 
The way you describe it above, the Tascam is feeding the Behringer.
That'll work to clock the Behringer to the Tascam... but it won't get signal to the Tascam.
Lightpipe is a one-way connection.
 

OK, it does seem to be working because I can record to Sonar from any input, Tascam or Behringer. And it works with the Tascam as the master or the Behringer as the master. It's just that I cannot hear myself during recording, only on playback.
 
I still have the older Tascam US1800 and US1641 from this series. They have a mix knob marked Input on one side and Computer on the other. On the US20x20 the knob's been removed. There's just a blank space there.
 
I'll probably go back to my US1800. I wanted the better preamps, but the US1800 is good. 
 
Oh, well. Another expensive box gathering dust. 

@Jim: Our posts crossed. As I said: I tried all ways, finally connecting both sides and that's how I have it now.

If I understand right, you'll have to go into  the TASCAM from the Behringer - it is a simply ADA (with pres) and not an interface to get the sound into the computer.  And you'll need the TASCAM going to the computer to control the TASCAM.  You should have the optical in enabled in the TASCAM soft mixer to get the sound in, then you can route it to the headphones.  The mixer app is one of those funky, dual page in and out formats.  I have to stare at it for 10 minutes every time I open it up and try to remember how the routing works (I don't mess with it much). 
 
The 3 tabs are like presets for what you want the hardware to do.  It can be used as a stand alone mixer for a live show, and it has presets for that, I believe, so you can set and forget.  But I imagine you want to run it as an interface, with the mic feeding the Behringer and that via optical feeding the TASCAM which goes via USB to SONAR.  That is two soft mixers without counting whatever the Behringer needs for routing.  It gets confusing.

On my Tascam us1641 you will not hear digital input on the headphones. This is because the monitoring is analog only,,, My 6i6 is the same. It's the one downside of bypassing the A/D. 
The direct monitoring is on the A side.