I do it all the time, along with mixing my own liquids. Here's a simple way to figure out what you ending up with:
Mixing two 5ml bottles to result in 10ml total.
5ml of 18mg/ml would be 90mg of nic
5ml of 12mg/ml would be 60mg of nic
90mg + 60mg would be 150mg in the final 10ml volume
150mg divided by 10mls would give you
15mg/ml strength.
You could just get into mixing your own stuff up, avoid higher strength nicotine.. it will kill you. Unless of course you are very chemistry/biohazard competent. Never buy the 100% strength (1000mg/ml), 120mg is more then enough to kill you and that would be about a drop of the stuff. I handle mostly 36mg/ml diluted down from 100mg/ml which I down with two pairs of nitrile gloves, tyvek gown and safety googles. A little over kill some may say, but if that bottle spills I rather not die. Keep in mind, I work for a big pharma and do this kinda stuff daily. Know your limits.