Well...it's weighted averages, actually, but you can make that sort of thing a lot easier.
Here's a method that everyone can use easily:
Say that you want to mix 20mL of 36mg/mL juice with 30ml of 18mg/mL juice...just because. What's your end nic concentration going to be?
Multiply your quantity of the first liquid, by its concentration: 20mL * 36mg/mL = 720mg total of nic in liquid #1.
Multiply your quantity of the second liquid, by its concentration: 30mL * 18mg/mL = 540mg total of nic in liquid #2.
Since you'll be mixing all you have of both liquids together, you'll have 50mL of liquid in total, but you'll be adding the total nic together, too; so you'll have 1260mg of nic in the one bottle.
Now, divide that total nic (1260mg) by the amount of liquid you'll have it in (50mL): 1260mg / 50mL = 25.2mg/mL nic concentration by volume.