Seeing as the heat death of the universe occurs once black holes have stopped emitting Hawking radiation, and BH’s life spans are tied to their mass, could a (very, very advanced) civilisation bring two or more together? Assuming nobody’s succumbed to proton decay before then, of course.
Bonus question: if so, what’s the longest timeline we could theoretically make? I’m thinking it would involve a black hole using all the matter in our local galaxy cluster.
I believe there have been multiple sci-fi stories written about such concepts. It would go to assume that any civilization still around in 10^43 years from now will have no choice but to live around black holes while harnessing their rotational energy. My current favorite series, Xeelee Sequence by Stephen Baxter, explores some possible endings of life in the long-term life cycle of the universe. There is also the final book of the Three Body Problem, which if I remember correctly, shows civilizations in pocket universes around black holes.
There is a YouTube video you may want to watch by Isaac Arthur. It mentions the possibility of combining black holes to elongate their life, and I won’t spoil the final answer for you. But like always, entropy prevails.