The War Domain cleric has the Channel Divinity option Guided Strike (PHB, p. 63):
Starting at 2nd level, you can use your Channel Divinity to strike with supernatural accuracy. When you make an attack roll, you can use your Channel Divinity to gain a +10 bonus to the roll. You make this choice after you see the roll, but before the DM says whether the attack hits or misses.
The Channel Divinity option War God's Blessing allows an identical benefit to extend to another creatures:
At 6th level, when a creature within 30 feet of you makes an attack roll, you can use your reaction to grant that creature a +10 bonus to the roll, using your Channel Divinity. You make this choice after you see the roll, but before the DM says whether the attack hits or misses.
If two War Domain clerics were in the same party, and one uses Guided Strike to add +10 to their attack roll, could the other War Domain cleric use War God's Blessing to add a further +10 to that attack roll?
Or would they not stack, as they are the same source (i.e. a War Domain cleric's Channel Divinity)?
Best Answer
They Stack.
Rules for combining game effects from the DMG:
Channel Divinity is not the name of these class features, at least, not the full name. Because Guided Strike and War God’s Blessing have different names, they stack.
As an example that falls in the same category that would more intuitively be ruled in the same way, imagine a Grassland Druid and a Cleric caste haste and warding bond respectively, on the same target. Naturally, we would rule that the target gets +3 AC, even though the two spells come from class features of the same name, “Spellcasting”.