I finish OIAL but I didn't see much that hinted at Eternum except the book being left out and the mention of the game. I guess they're not related that much, just same world and some villains likely connected.
Haha oh common they are not kids apparently,they are mature and married couple,why would they play video games?, but if you want with this angle cameo maybe eternum could be in timeline after 15 years of events of Oial and children of mc and his waifu would be playing it.
adults play games too I'm jjust saying they would hear about that corruption in Eternum and not want to join the game. They would want to avoid that especially after the cult they fought.