I'm intersted in this as well. I don't think you can because like Legendary creatures and planeswalkers you can't use their abilities. But with legends you'd get an enter the battlefield effect.
Can I tap the second Mox Opal before I have to sacrifice it due to the Legend Rule?
No. When an object enters the battlefield, state based actions are preformed before you receive priority. As a result, the second mox opal will never be on the battlefield (assuming no mirror gallery shenanigans) at a point where you can tap it for mana.
No. Even though the Opal's ability is a mana ability, you normally need priority to activate it, and you don't get priority between the second Opal entering the battlefield and the "legend rule" state-based action putting both Opals in your graveyard.
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It is at least mechanically possible that some effect would let you "tap a 'second Mox Opal' before it dies to the legend rule (after entering the battlefield)." It depends on how the Opal gets onto the battlefield. However, I checked, and there is in fact no such effect at this time. (One card 'almost' has the wording for this to be possible, and it has been errata'd a few times.) So, for any currently possible game situation, if a Mox Opal enters the battlefield as a Mox Opal, it will be annihilated by other Mox Opals before you can tap it for mana if the legend rule is applying. Mirror Gallery is your only out.
Now I should say that the legend rule doesn't cause permanents to be sacrificed. It only causes permanents to be put into graveyards.
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No. When an object enters the battlefield, state based actions are preformed before you receive priority. As a result, the second mox opal will never be on the battlefield (assuming no mirror gallery shenanigans) at a point where you can tap it for mana.
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Now I should say that the legend rule doesn't cause permanents to be sacrificed. It only causes permanents to be put into graveyards.
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