Reaper applies to necromancy cantrips

The cantrip must normally target only one creature. When the positioning condition is met, the same casting can affect a second creature.

Improved Reaper applies to leveled necromancy spells

At 17th level, the same idea expands to necromancy spells of 1st through 5th level that normally target only one creature.

The targets must be close together

Both creatures must be valid targets and within the feature's required proximity to one another. The second target is not a free arbitrary creature elsewhere on the battlefield.

Spell rules still apply to each target

Attack rolls, saving throws, immunity, range and other requirements continue to apply as appropriate.

Consumed material components can double

Improved Reaper explicitly requires material components for each target when the spell consumes those materials, preventing one costly component from paying for two consumed effects.

No 6th-level or higher Improved Reaper

The 17th-level feature is limited to necromancy spells from 1st through 5th level. High-level necromancy spells do not automatically gain the extra target.