2WD & 4WD need to be together.
Like Martin says there is not enough gap between ability's.
Whats the point of having one 4WD heat in which the lead car laps the back marker 9 times in 16 laps? Its just not fun
You can only split 2WD & 4WD if it can be made so there is a lap or so difference per heat.
On a separate note, I run race control with Roger at Bury most weeks so i know the system. So if you guys need any help with it next week let me know.