Not a problem but if you mix two of diff nicotine levels you will not know what you have after you mix them.
Atually, mixing 2 is easy formula.
i.e. 18mg + 12mg /2 (in equal amts) = 15mg nic outcome
I have to do this all the time, buy 2 bottles and mix because I enjoy vaping at around 15mg.
If you have juice that you like, and would order again, but it's too strong, you just order a 2nd bottle in lower nic and mix:
24mg + 6mg /2 = 15mg.