I assume you're capturing with display capture and you are asking how to insert two different parts of the display to OBS.
There are 2 cropping methods: adding a crop filter or adjusting the red borders of a source with the mouse while holding the ALT-key.
If you did your crop with the crop filter, remove this crop filter from the display capture source. Instead, use the second cropping method to crop: move the borders with the mouse by holding down ALT and move the red circles.
Now, add another display capture source, and choose "add existing" while adding it. This way, you create a reference to the first and are able to get a second copy of your display and crop a different part out of it. This would not be possible if you have a crop filter at the original display capture, because filters are inherited by the "add existing" function. Moving the borders, on the other hand, is specific to each reference, and you are able to create many different crops out of the same original source.
If you need to apply different filters to your duplicated display capture sources, don't apply filters directly to the sources but put the source where you want the filter into a group and apply the filter to the group. This way the filter isn't inherited by the monitor capture references.