Keep them as double rock. If you continue to play and even walk larvitar as a buddy you'll gain enough candy for another Tyranitar. Maybe you see a Tyranitar raid and others show up for it since they likely also need extra larvitar candy to power their new ones.
Maybe you hatch a larvitar in a double candy egg event, you likely have a pantry full of 2k egg...maybe they get replaced with 5-9 10k eggs