In my own experience, fitting two mic cables into a single miniXLR connector is a PITA.
To give you some visual idea of what's involved,
start here and read that post as well as the 4 posts following. Basically, you would do what's shown in those pictures except you would be squeezing both mic cables into one miniXLR connector.
As for the wiring that Core-Sound uses, I have no idea what the wire/pin assignments are... but you have two options to find out: email Core-Sound, or email Sound Pros. (I believe Sound Pros knows the Core-Sound pin assignments, as Sound Pros offers
a Core-Sound connector mod.)
A good (and less expensive) source for miniXLR connectors is
Trew Audio, and they have a store in Toronto.
Since it's not customary for you to do this sort of DIY, you might want to buy 2 connectors.
Good luck!
P.S. I know you said you didn't want to, but you could always send them back to Core-Sound, or even to Sound Pros, and get them to do it. Sure, it might cost more, but it's no sweat on your part.