In Omaha hi low, will holding an Ace and a Deuce card automatically give you the "nut low" hand in the event of a qualifying low hand?
A2 will be nut low when:
It might still be nut low in other cases, for example where either the A or the 2 completes a wheel with 4 board cards.
But in most--not all--cases, nut low is the having in your hand the two lowest cards not on the board. Pairing your low cards is a bad thing in Omaha. That's why hands like A23, A24 are much better than those with "naked" A2--the extra low card is "counterfeit protection",
You can only lose the low hand if you get counterfeit (when a low card hits the board that also pairs one of your low cards), so that you don't hold the nut low on the river anymore.
If there is a low hand possible, and no A or 2 on the board, you always have the nut low. At worst, you will split the low pot if someone else also has a A2XX hand. With that said, it is possible to win a quarter of the whole pot (50% of low pot and 0% of high pot).
You have A2, villain has A3.
Board is: 24567.
That gives you a low: A2456
Gives villain low: A2345
Basically, whenever one of your cards pairs, you might be in danger.