Adding 50ml of 36mg to 50ml of 12 will give you 100ml at 24mg, according to
ejuice Me Up.
Edit: If you'd want to do it in smaller portions the math is easy because you're just doubling.
In other words, if you pull off 20 ml of the 12, mix it with 20ml of the 36, you'd have 40ml at 24mg and could store the rest for a later date if that would work better.