How do you get Quad from a Quad Capture UA55?

I own a Quad Capture UA55. While its called a "quad" it will only record two analogue inputs simultaneously. I understand that to utilize the "quad" ability you need to input an additional two digital signals.
How is this done?
 
Where do I find a digital signal?
 
Would taking the digital output from a second UA55 to the digital input of the first UA55 allow me to record four inputs simultaneously?
 
I have looked on the Roland website but can find no clues so I hope someone can help.
 
Thanks and cheers