You can steep them mixed, or separate, it doesn't matter. From a mixing perspective, you'd do well to get a couple of mixed bottles steeping though. That does sound like a good plan.
Oddly, sometimes mixing a steeped liquid with a steeped liquid creates an additional steeping time for those two mixes to blend. Make
sense?
If you were mixing something water or alcohol base, the mix would be pretty instantaneous, but we are dealing with viscous liquids.