Tooling Split

Hi, I'm trying to create a two part female mould for a wishbone.

I started by forming the wishbone surface but when I try to use  tooling split it won't let me select sketchs/ surfaces that I want. I've tried creating other surfaces off the part which I can select but then I get a cannot knit together error.

Any help would be greatly appreciated.

SolidworksPlastics And Mold Design