Are you planning on adding a set of RCAs, or using one of the existing pairs to connect the Crack? I was going to do the same thing, but then thought I'd put in a switch and a dedicated set of RCAs. I have all the parts now, but I haven't figured out where to place an extra set of RCAs, so I'm on the verge of doing what you're suggesting: just splitting the output, sending the other to RCA set 3.
If you're adding a fourth pair, where on the main board do you plan to do it?
Regards,
Derek