Decware ZSB please help

Hello all

  The ZSB box I have is the one yogiboy4 listed above. If you can look down on the website page for this box you can see only a hand drawn schematic but you can order this box with a switch option but mine does not have this so I wanted to add it so I can run two sources at one time.  Right now when I try to run two sources only one source works and the other seems to not work. only one works at a time. 
Thanks 

Sorry let me clarify I want to be able to switch from one source to the other source. Not run both at the same time. If you look a Steve's drawing for the ZSB it show just running one source and four amps but if you have the switch you can use both of them by switching them. 

OK OP my only question; do you have 2 (two) RCA inputs or one?

If you have only one pair or L/R RCAs add a second set. Cut the wires and leave pig tails on the original switch 1/2 - 1" long, from the input RCAs

DON’T cut them to short.

You need THREE dual poles and a two position toggle switch. The center 2 poles is the feed for the two pigtsils you left on the original switch.

The other two pairs of RCAs (L/R) goes to either side of the feed wire poles (center).

https://www.littelfuse.com/~/media/images/technical-resources/commercial-vehicle/switch-pole-throw-d...

You can have an OFF position by adding a 3rd center throw or it could combined both A and B positions (not usually a good idea)

So
A or B
A off B
A both B

Regards