60 ml of 36 mg/ml would give you 180 ml of 12 mg/ml or twice that of 6 mg/ml.
60 ml x 36 mg/ml = 2160 mg total nicotine. 2160 mg nicotine / 12 mg/ml = 180 ml juice.
36 mg/ml should leave you enough room in your juice to get all the flavors and stuff in. Look at one of the calculators available (ejuicemeup or the spreadsheet, there are many available). Plug in the concentrations for the base you are considering, plug what you want your final pg/VG ratio to be, and what percentage of flavors you are thinking of using, and see if what you have in mind will work.