I'm wondering what are the considerations for having multiple detectors instead of a single large one! Does anybody have an idea? is it just because it's easier?

The primary detector is the wide-field instrument, which, believe it or not, takes pictures with a wide field of view. The secondary detector is the coronagraph instrument, which uses a bunch of funky filters to block out the light from the star to see the halo around it.

Edit: These are the detectors, which are the cameras. There is only one mirror assembly (with two mirrors) that focuses light into these instruments.