great, sounds promising but i don't follow
You'd need to make sure the existing symbols are reset before that
You mean reset the designators? How can I expect Altium to match them both together if I reset them? Normally I would manually set them as U1A, U1B
Yes, if the designators are already set then Altium won't annotate them and won't assign them to the same package. If you reset just those two designators, and then annotate the schematic with "Complete Existing Packages" set to "Whole Project" it should see that those parts can be fit into one physical package and annotate them that way (as long as one is A and one is B). If you had a bunch of the same part all with reset designators then it might pick two other instances to make a complete package, but should still try to make complete packages across schematic sheets.
If the parts are already linked to separate PCB components then even resetting them and re-annotating them might not get what you want, which is why I suggested it might be easier to delete and re-add one of them.
If you're manually annotating your schematics, then I'm not sure how effective the "Complete Existing Packages" option is. This is an area where Altium can be mercurial and fussy, and sometimes you just have to try things until one of them works