You can tack hushkit installations and seat configurations on to a C/D check, but not the other way around. You can't start a C/D check if the plane is in any other kind of maintenance (including manual B-checks). For that reason, I wait until a plane is in C/D-check and then I add hushkit installation to the check, which both adds 14 days to the duration of the check and pushes the date of the next C/D-check out another 2 weeks. If I'm up against the deadline, and the C/D-check is less than 15 days away, I'll trigger it manually and then add the hushkit to the check.