The rules are pretty clear. You don't break Sanctuary unless you make an attack or cast a spell that affects an enemy.

As for it being OP, you're making a cleric not attack enemies or cast offensive spells, to maintain Guardiões do Espírito. It's not massive damage, and given that the cleric using it is doing nothing else, enemies don't even have any reason to go near them. Sure, that cleric is hard to attack, but enemies have no reason to attack them, and every reason to attack every other member of the party. If for some reason you really don't want that specific cleric to be attacked, this becomes useful. But if you compare it to the cleric using Spiritual Guardians, Arma Espiritual, e using his action every turn, his usefulness as a character doesn't measure up.

Guardiões do Espírito, like several other spells, is a powerful spell because you can keep doing other things while you use it. It adds a bit of ongoing damage, and in a long battle you get a lot of damage for a minimal resource investment. Using it like this, you're putting in a lot of resources for not all that much damage.

As of the 2018 errata, if an enemy takes damage from your guardiões espirituais spell, your santuário spell ends

A descrição do santuário spell (PHB, p. 272) originally said:

If the warded creature makes an attack or casts a spell that affects an enemy creature, this spell ends.

O Errata do 2018 PHB added "deals damage to another creature" to the list of conditions that end the santuário spell. The description now reads:

If the warded creature makes an attack, casts a spell that affects an enemy, or deals damage to another creature, this spell ends.

Mesmo que o guardiões espirituais spell doesn't directly affect an enemy creature when cast, it does deal damage to them:

An affected creature's speed is halved in the area, and when the creature enters the area for the first time on a turn or starts its turn there, it must make a Wisdom saving throw. On a failed save, the creature takes 3d8 radiant damage (if you are good or neutral) or 3d8 necrotic damage (if you are evil). On a successful save, the creature takes half as much damage.

And thus, the santuário spell ends as soon as an enemy takes damage from guardiões espirituais.

Even before the 2018 errata, the intent was that damaging an enemy would end the spell (though this was not reflected in the description of the spell). At the time, 5e rules designer Jeremy Crawford unofficially stated this on Twitter in 2016:

You first cast spirit guardians and then sanctuary. Does sanctuary end when spirit guardians deals damage to enemies?

RAW: Sanctuary isn't stopped if an already-cast spell deals damage. RAI: A damaging activity ends the spell.

RAI means errata or a future edition would possibly change RAW, right? Thanks for the answer

That's correct

not sure I understand this: by RAW it's possible, but you guys "patched" it with RAI but haven't published an errata about it yet?

We haven't changed the spell.

But you will right? Otherwise the difference between RAW and RAI is lost to me

It works as written, so you can confidently use RAW. If you're the DM and prefer the RAI, change it.

Clearly, the 2018 PHB errata has changed the spell to work as intended.

While powerful, it's within RAW and is not that dissimilar to using other methods of defence, such as Full Defence action from a cleric which is concentrating on Spirit Guardians, or Cutting Words.

It's a good enough spell that you could base an entire build around it- Human Cleric with War Caster and a +3 con bonus, a Shield and Plate Armour. Assuming the creatures attacking him have +5 to hit, they only have a 9% chance of hitting him with each attack, and a 9% chance of breaking his concentration if they do hit.

So planning the Sanctuary combo out between two characters isn't really any more effective than what a single well planned character can do on his own.

The only possibly broken part here is the raw power of Spirit Guardians. Seen it in three games now and every cleric is using it. But then again, that's not unlike Wizards and Fireball and Warlocks and Eldritch Blast.

Sanctuary: You ward a creature within range against attack. Until the spell ends, any creature who targets the warded creature with an attack or a harmful spell must first make a Wisdom saving throw. On a failed save, the creature must choose a new target or lose the attack or spell. This spell doesn’t protect the warded creature from area effects, such as the explosion of a fireball. If the warded creature makes an attack or casts a spell that affects an enemy creature, this spell ends.

Spirit Guardians: You call forth spirits to protect you. They flit around you to a distance of 15 feet for the duration. If you are good or neutral, their spectral form appears angelic or fey (your choice). If you are evil, they appear fiendish. When you cast this spell, you can designate any number of creatures you can see to be unaffected by it. An affected creature’s speed is halved in the area, and when the creature enters the area for the first time on a turn or starts its turn there, it must make a Wisdom saving throw. On a failed save, the creature takes 3d8 radiant damage (if you are good or neutral) or 3d8 necrotic damage (if you are evil). On a successful save, the creature takes half as much damage.

So if Cleric 1's Spirit Guardians afeta an enemy creature (which happens as soon as an enemy moves into it), then Sanctuary on Cleric 1 ends.

Edit: Hmm..down votes? As a person that has been a D&D cleric for a very long time, I would love to have a good argument to give the DM as to how I can accomplish affecting the enemy while maintaining Sanctuary. No DM I have ever played with would ever allow such a thing. Please leave a comment so that I can accomplish this as well.