Is this something that profiles need to be able to do?

If so, perhaps the merged control has a new title, while its old titles are demoted to p elements?

Are controls to be merged, called by ID? Does the new merged control have its own ID as well?

Does the merged control contain all the properties, parts and contents of the controls, which have been merged?

Can subcontrols or parts be merged? Can controls be merged with subcontrols, etc?

It might be argued that merging controls is precisely not what OSCAL should encourage, in which case the requirement should be placed far far away.