Want to merge my UK and USA Amazon marketplace

We have initially opened a selling account in UK. After a while, we opened a new selling account for the USA (using two different email addresses). Both UK and USA companies are different entities and have different Amazon accounts. We would like to merge the UK and USA account to get benefit of our existing UK based listings.

We can’t figure out how to merge the accounts as when we try to merge the account it gives an error which is "Cant merge the account. There in pending in-registration".

Can someone please help me to merge both the accounts?