No trigger coil is required on a slave coil (all transistors are triggered by the same trigger coil on the master coil) so you don't need different guage wires. You can use just one wire (single filar) for the coil, though you may as well use at least 2 wires to make a bifilar coil just like you did for the first coil. You can add more wires if you prefer... any more than 5 may be a bit excessive though, unless you are making VERY big coils! lol
The schematic below shows how to hook up a single filar coil as a slave though just add the part in red for however many wires/coils you want to use.
This can also be used for making a trifilar Master Coil with two wires powering the coil and one for the trigger coil.
Good luck, and feel free to ask any questions
