Union function has been recently added to Data Designer, in both SFDC and NXT editions. This function combines the two datasets in a way that all the similar type fields automatically get unified into one dataset. If there are any other fields in the two datasets that are not similar, you can select and add them based on your requirement.
Example Business Use Case: For instance, you want to combine the data of two objects (with similar fields) that have the data from Europe and US region customers.
If you wish to learn more, following are the doc resources: