I've honestly never heard the word “semipiternal” in my life – and
I consider myself a fairly well-read person. Dictionary.com says there is a
small semantic difference:
Sempiternal is useful for describing something endless, especially when
you want to use an impressive word. Although it's often used the same way you'd
use the word “eternal,” in philosophy there is a distinction between those
terms. “Eternal” implies something that is infinite outside the bounds of
time, like God, while sempiternal is a more earthbound way to talk about
forever. But you should feel free to tell your annoying little brother, “Your
presence in my room feels sempiternal.”
All I can say is that very few people will know what it means… 