I run them separately..it's lights OR heat.
The grip heat has a hi/lo setting. Both consume the same amount of power. The 'lo' setting simply runs through a big ceramic resistor...so even when they are set to 'lo' the current draw stays pretty much the same.
So...'lo' heat
and lights won't work, either.
I don't run lights during the day (my taillight illuminates in either 'heat' or 'light' mode..if I have a bulb in there), I DO run grip heat all day (this time of year anyway). I switch between the 'hi/lo' settings 'cuz 'hi' gets too hot sometimes.
On banzai runs back to staging in the dark..my hands just freeze.
An either/or setup would run with the OEM light coil. I ran an Electrex coil that way for years..currently have one'a Inda's coils.