Is it worth duplicating a table for different types of an entity when one should be unique?
Some people might wince at this, but if you have at least one column that distinguishes sale opportunities from customer prospects you could make this column nullable and create a unique index that includes the customer ID and this column. 

Db2 allows only one NULL value in a column in a unique index (in other words, from the uniqueness point of view all NULL values are considered equal). Thus you will store a NULL in this column for "possible customer" and something unique for each "possible sale", and Db2 will ensure that only one "possible customer" (i.e. NULL) value is present for a given customer ID.