you can actually mix any juice together that you want regardless of nic level. If you mix one that has 16 mg with one that has 24 mg in equal parts, the resulting liquid will be 20mg/ml.
Quick sample
1 part 24 + 1 part 18 = 21 mg
1 part 24 + 2 parts 18 = 20 mg
Oh yes, you can mix juices, no problem.
Flurfel explained 50/50 mixes above, but you can mix in any ratio. There are calculators that will tell you what the resulting mg/ml is, if you need to know.
The ejuice app is one of these. You can also do it manually with just a little math.
Say you have juice A at 12 mg/ml and juice B at 18 mg/ml, and you mix them at a ratio of 4 parts A to 1 part B:
Mg of nicotine from Juice A: 4 ml * 12 mg/ml = 48 mg
Mg of nicotine from juice B: 1 ml * 18 mg/ml = 18 mg
Total mg of nicotine: 48 mg + 18 mg = 66 mg
Resulting mg/ml: 66 mg in 5 ml = 66 mg / 5 ml = 13.2 mg/ml
You could also figure out the PG/VG ratio if you needed to, but it's not usually all that useful to do so.