I'm playing a Grave domain cleric with the Metamagic Adept feat and Twinned Spell as an option. My question is if I cast a healing spell like cure wounds on a single target at 0 hit points and use my feat to twin it, do they receive the maximum healing for both iterations of the spell or just one and rolled healing for the second? Mechanically speaking it's still one spell slot and the healing would be happening simultaneously.
This doesn't work.
Twinned spell reads:
When you cast a spell that targets only one creature and doesn’t have a range of self, you can spend a number of sorcery points equal to the spell’s level to target a second creature in range with the same spell (1 sorcery point if the spell is a cantrip). To be eligible, a spell must be incapable of targeting more than one creature at the spell’s current level. For example, magic missile and scorching ray aren’t eligible, but ray of frost and chromatic orb are.
Emphasis here is mine, but the gist of it is that you can't use twin spell in this way at all- you must explicitly target a second creature.
The nature behind your question
Circle of Mortality reads:
At 1st level, you gain the ability to manipulate the line between life and death. When you would normally roll one or more dice to restore hit points with a spell to a creature at 0 hit points, you instead use the highest number possible for each die.
Circle of Mortality has no restrictions on how many times it can trigger per round. If you presented the same scenario and targeted two creatures at 0 HP to make it a legal use of twin spell, instead, both creatures would receive the benefit of Circle of Mortality.
You can't target the same creature with both instances of a twinned spell
Have a look at the wording of the Twinned Spell Metamagic:
When you Cast a Spell that targets only one creature and doesn’t have a range of self, you can spend a number of sorcery points equal to the spell’s level to target a second creature in range with the same spell (…)
To twin a spell you need to target another creature, the text of Twinned Spell explicitly requires the creatures to be two different ones.
If you use this to heal two different creatures, both must be at 0 hp in order to receive the benefit from the Circle of Mortality
Circle of Mortality says:
When you would normally roll one or more dice to restore hit points with a spell to a creature at 0 hit points, you instead use the highest number possible for each die.
This includes no restrictions on how often it can be used or regarding the number of targets it can affect simultaneously, the only requirement is that the creatures targeted needs to be at 0 hit points. So there's nothing that would prevent this from working together with Twinned Spell but both creatures must meet the requirement separately.
So if you apply Twinned Spell to Cure Wounds, both creatures will receive maximum healing if they're both at 0 hp at the time, if just one is, then only that one benefits from Circle of Mortality and for the other one you roll the healing as normal.