Are you wanting to add the columns from one dataset to the other. This is what Phil means by "JOIN-like." If so, do they have the same number of rows? If so, you can simply use system.dataset.addColumn.
If not, do they share a common column with a unique identifier that can be used to stitch the data together.
Here is an example:
• Combining two datasets based on a primary key
On the other hand, are you trying to simply add the rows from one dataset to the rows of another? This is what is meant by "UNION-like."
For this, you can simply use system.dataset.appendDataset
The answer could be simple or complex depending on your use case, and the possibilities are quite diverse, so if the above examples are not what you are looking for, then be more specific, and I'm certain somebody around here can give you the example you need.