I'm trying to figure out what I'm missing if this combo actually works.

Adam has on the field and casts targeting Sharuum.

Here's where I get confused

Sharuum 2 ETB triggers, they're wanting to return the original Sharuum claiming legend rule. Here's what's going through my mind:

A:Cast metamorph targeting Sharuum (Pass priority)
SBA's Check
N: (No response pass priority)
SBA's check
A: Sharuum 2 enters triggering the ETB effect. Before the effect goes on the stack there has to be a target. Target is chosen (Pass priority)
SBA's check triggering legend rule

It's looking like there is no point in time in which 1 is in the yard with the other's ability not on the stack targeting a different artifact. What am I missing?

Hi,

Thanks for your question. Your issue is that you things backwards. If a player would get priority, the first thing we do is check state-based actions, then we put triggered abilities on the stack:

116.5. Each time a player would get priority, the game first performs all applicable state-based actions as a single event (see rule 704, “State-Based Actions”), then repeats this process until no state-based actions are performed. Then triggered abilities are put on the stack (see rule 603, “Handling Triggered Abilities”). These steps repeat in order until no further state-based actions are performed and no abilities trigger. Then the player who would have received priority does so.

So after the second Sharuum enters, before anyone gets priority, we first check state-based actions. This causes one of the Sharuums to die due to the legendary rule. Then we put the Sharuum's enter the battlefield trigger on the stack, and since that Sharuum or Metamorph is in the graveyard, it can be targeted by the trigger.

Thanks,
Nathan Long