Fujifilm X-H2 has a 40.2MP APS-C (23.5 x 15.6 mm ) sized BSI-CMOS sensor and features X-Processor 5 processor. On the other hand, Fujifilm X-H1 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features X-Processor Pro processor.
Fujifilm X-H2's sensor provides 16.2MP more than Fujifilm X-H1's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Another similarity between these two cameras is that both Fujifilm X-H1 and Fujifilm X-H2 sensors lack anti-alias (Low-Pass) filters. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the X-H2 and X-H1 sensor size comparison.
Fujifilm X-H2 and Fujifilm X-H1 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Fujifilm X-H1 has 65% larger pixel area (15.28µm2 vs 9.21µm2) compared to Fujifilm X-H2, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.