I feel like this may not need to be said. But if you are mixing 2 bottles. Make sure you know the amount in each bottle. If you have 2 15ml bottles one at 3 one at 0. Yes it will be 1.5. but if you have a 15ml bottle and a 30ml bottle. Mixing them will not put it at 1.5

Yes, it's perfectly fine to do so. Shake and vape my friend.

If you do a 50/50 mix of two juices of varying nic levels, just add the mg together and divide by half. So 50% 18mg and 50% 6mg would result in 12mg. In your situation, if you add 50% 3mg and 50% 0mg, you are correct that the result would be 1.5mg, as long as its exactly half of both being mixed together.

Edit: divide auto-corrected to decide...

Of course it's fine,. Shake it up and vape it..damn.

Diff nic juices should be okay. Consider that some people report higher nic = more bitter/stronger flavor. So the 1.5ml should be in the middle.

Its doable, but ~~6mg~~ 0mg + 3mg ~~0mg =~~ = 30mg! so be careful.

Edit: Math

Edit2: More math