It is likely somewhat to do with the nic level. That said, casa is very subjective with steep time. It changes over a few weeks more than just about any juice ive ever tried. If he has just recently gotten the bottle, it might be worth giving it a little time.
Now as far as mixing the two mg levels, in theory, it should give you the same product. I know at least for me, this dosent always work out...even though it should.
Could you tell me what the born on date of the 24mg bottle is?
January 31st, so we still have a ways to go for steeping.