To achieve 6mg, you can decant 10ml of the liquid from the bottle and add 2 nic shots instead of one, which will have a small impact on the flavour strength but in my experience liquids made up to 6mg in this manner still taste absolutely fine (any decent shortfill will be over-flavoured to account for the first nic shot anyway)
12mg from a shortfill is only really achievable with 72mg nicotine solution, which is now illegal to sell (thanks TPD) - not to say there aren't those out there still selling it but I can't really advise any further on that I'm afraid.