Two Clerics who cast Spare-the-Dying on eachother, become immune to death.
The effect is similar to the Regeneration spell.
The cantrip should Stabilize (remain 0 hp). Maybe with a casting time of 10 minutes, it can additionally heal a Stable target to 1 hp.
Well, think about that logically. Let's imagine a scenario:
Monsters act on initiative 20
Cleric 1 acts on initiative 15
Cleric 2 acts on initiative 10
Let's assume the party has no other healing (two clerics is a lot of healing already)
on the Monster turn, Cleric 1 is dropped.
Cleric 1 makes a death save - we don't care if he succeeds or fails.
Cleric 2 now takes her turn and casts Spare the Dying on Cleric 1, who now has 1 hit point.
It's now the Monster turn again. One monster goes for Cleric 1, possibly dropping him again. All Spare the Dying is doing here is typing up party resources, stopping them from healing Cleric 1 properly. Far from a situation where two Clerics with the cantrip are immortal, two clerics relying on this cantrip to keep them up are *extremely vulnerable*: All it'll take is two monsters connecting with a blow or one area-of-effect attack and they're both going to go down.
Spare the Dying isn't a cantrip to use when one of the PCs falls down in melee - it's to use once (or while) you get them out of immediate danger, trusting that they can sort their own healing out or play dead long enough to survive the fight.
I'd use it on a PC who has been downed by ranged attacks or (non-persistent) area-of-effect attacks. And if I had any choice in the matter, I'd probably use Healing Word instead.