A better male "equivalent" for Nechama would be Nachum.
Another reason for similar sibling names: There was an ancestor named Chayim. Descendants wanted to name their child after him, but the child was a girl, and they had no way of knowing if they would have another chance, so they named the girl Chayah. Then, several years later, they had a boy, Given this opportunity to "do it right", they called him Chayim.
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