Create an alternate key with two attributes in entity and use it as a unique key in the Entity validators.
this link will be helpful..
The following example illustrate how to create a unique key using two or more attributes/fields
Scenario: Implementing Unique Key on ManagerID & Location ID in DepartmentEO(Department Table)
Step#1: Open the Desired Entity Object “DepartmentEO”. Go to Overview tab & Click “General” Finger tab.
Step#2: Expand “Alternate Keys” section & click “+”.
Step#3: In the Pop-up wizard, Enter a name for alternate Key “Unique_Manager_X_Location”.
Step#4: Select the desired attributes/fields from available list & Shuffle to right side.
Step#5: Now go to “Business Rules” finger tab.
Step#6: Select “Entity Validators” in the list & click “+” to add a new entity level validation rule.
Step#7: In the Pop-up, Select “Rule Type” as Unique Key
Step#8: In the “Rule Definition” tab select the key “Unique_Manager_X_Location”created.
Step#9: Now go to “Failure Handling” tab, and click the Magnifier Icon .
Step#10: If the key is not already created then in the “Select Text Resource” Popup, Using the functional design document, Enter display value, Key and Description. And click “Save & Select” Button.
Step#11: Now Click “OK”.