The ability from the Death Domain Cleric is Improved Reaper:
Starting at 17th level, when you cast a necromancy spell of 1st through 5th level that targets only one creature, the spell can instead target two creatures within range and within 5 feet of each other. If the spell consumes its material components, you must provide them for each target.
Nowhere in the ability does it state that spells with a range of 'Self' are excluded. This differs from the Sorcerer's metamagic Twinned Spell:
Twinned Spell. 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 for Twinned Spell, a spell must be incapable of targeting more than one creature at the spell's current level.
Assuming that the caster is within 5 feet of an ally, could Improved Reaper be used to cast two Spirit Shroud spells with only one person concentrating?