You related two objects together by adding a custom field or type "Master-Detail" or "Lookup". So if you want CustomObject2 to be the child of CustomObject1, create a custom field or type "Master-Detail" on CustomObject2, relate it to CustomObject1 and save the custom field. That will create a Master-Detail relationship between CustomObject1 (master) and CustomObject2 (child).
You related two objects together by adding a custom field or type "Master-Detail" or "Lookup". So if you want CustomObject2 to be the child of CustomObject1, create a custom field or type "Master-Detail" on CustomObject2, relate it to CustomObject1 and save the custom field. That will create a Master-Detail relationship between CustomObject1 (master) and CustomObject2 (child).