Keroles,
if it would be allowed to combine operations by omitting one it would cause issues in scheduling. Think about the nightmare to get all the involved objects into sync. That is also the reason why an original order can only be part of a combined order once at a time. So you can not combine operation 10 in combined order #1 and at the same time combine operation 30 in an combined order #2.
I only see a chance that you change your master data so for example you get operation 10 and 20 into one production order and 30 and all the following into another production order. I know that's also a nightmare but based on the restrictions in OC I don't see another chance.
regards
Stefan