Absolutely a coincidence. There was no coordination between us authors on this.Today I'm wondering if there's a connection, considering two out of the three most recent TOS novels have Eternity in their title.

Just a coincidence. I didn't know the title of Greg’s book when I started working on Echoes…, whose title was inspired by the famous quotation of Marcus Aurelius, “What we do in life echoes in eternity.”
Every thing material shall soon vanish, and be swallowed up in the matter of the whole. Every active principle shall soon be resumed into the intelligence and cause of the whole. And the memory of every thing shall soon be buried in eternity.
Then it sits proudly alongside In the Pale Moonlight.IIRC, that quotation is often misattributed to Marcus Aurelius, but originated in the movie Gladiator.
Marcus's sentiments in his Meditations were closer to @Greg Cox's final title:
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