Consider you have two forms "Department" and "Employee", you can import data for both the forms via the "Employee" form itself, if "Department.Name" is looked-up here as SINGLE SELECT. For this, go to the live mode of the "Employee" form and select "Import Data" from "More Actions" menu. 

Upload the data file(having both "Employee" and "Department" details) and click "Next". ZohoCreator will analyze the file data either by field name, if provided, or by order in which the fields appear in the form and show the matched fields in "Step 2". Here you can verify the matches and make your changes accordingly. ZohoCreator will also check for "Department" field names and matches it too. If not, you can anyway select it from the fields listed in the preview drop-down. 

After making the necessary changes click on "Import Data". This will import the data and show the import summary for both "Employee" and "Department" separately at Step 3. The data will not be added to the "Department" form if it already has the department name.