You can certainly do more than one herb in any type of infusion or extraction (water, alcohol, vinegar, oil). Using just one herb does give you more flexibility later on for blending pre-made preparations but this is not necessary, as long as all of the different herbs have similar extraction times and requirements.
Fresh plantain can be infused in oil, in the same way to Comfrey. I’d just recommend letting it wilt for a few hours before putting it in oil. This oil extraction would also be best with some heat applied, to evaporate some of the water from the herb to prevent the oil spoiling.