How to Connect Sunn Beta Dual Inputs?

[daustinhall]

Looking at this http://tagboardeffects.blogspot.com/2016/03/sunn-beta-preamp.html

The page says to do the two channel version you just build the single channel twice and use the provided daughter board to sum them. That sounds great, but it doesn't show how to modify the inputs for the signal to go through both channels.
Even though the outputs are summed, if one channel doesn't have an input, what is being summed?
 
I guess my question boils down to, how do I split the guitar to go to both channels? Do I have to do something like the buffered AMZ splitter, or is it more simple than that?

Thanks