Complex sentence: The first concerto reveals that Paganini's technical wizardry was already fully fledged; in particular, contemporary audiences must have gasped at the quite extended passages of double-stop thirds, chromatic in some places but also sometimes in harmonics.
Split into simpler sentences: The first concerto reveals that Paganini's technical wizardry was fully developed by this point. It is thought that, in particular, contemporary audiences must have gasped at the quite extended passages of double-stop thirds, both chromatic and in harmonics.