Are you allowed to say the beracha for thunder on its own if you didn’t see the lightning, or can you only say the lightning and thunder berachos together?
It is fine to say a berachah on thunder on its own, even without seeing lightning.
In this case, one can actually choose to make either “maaseh bereishis” or “shekocho u’gevuraso malei olam.”
When making both berachos separately, one for lightning, and another for thunder, one would make maaseh bereishis for lightning, and shekocho u’gevuraso for thunder.
Usually, however, one sees/hears them together, and one berachah is made for both.
See Be’er Heitev, Orach Chaim 237:1; Eliyah Rabba explains that unlike lightning, thunder is heard ‘throughout the world,’ making the kocho u’gevuraso blessing a viable alternative to maaseh bereishis; Peri Megadim explains that it is better to make one blessing than two; see also Kitzur Shulchan Aruch 60.