party number and customer number

Hi ,

We are facing an issue in receivables. We are having separate party number for same customer in different operating units. We create duplicate customer and identify the company by attaching short form of company name in the alternate name field.

Now  we are facing an issue wherein for some customers party number is same in two or more operating units. we want to separate this and want to have separate party number for each operating unit. I try to create a duplicate with a new party number  and try to do customer merge. But the system is throwing a error and it says account merge is not possible.