merge 2 iCloud storage for 2 devices i own

If you have an iCloud storage plan of any size - including the free 5GB plan - all your devices signed into the same account share the same iCloud space and have access to all the content.

Note that the 5GB is not on any of the devices, it is the space on iCloud.

What's the difference between device storage and iCloud storage?

Content from one Apple ID cannot be merged or transferred to another Apple ID.