These 2 blessing are recite merely if you original see the lightning & then hear the noise, or associate versa. If, though, you concurrently see lightning & hear noise, you now say one of the blessings, which still one you wish, & that cover together.
Single blessing suffice for every one the noise heard & lightning see as extended as the fumes have not empty. If the sky obvious & a new tempest approach, you have to declaim the blessing one more.