You have a live data model with two dimensions: Firstname and Lastname.

To combine two dimensions, Firstname and Lastname, into a single dimension displaying as Lastname, Firstname in a live data model, you should create a calculated dimension in the data model. This calculated dimension will concatenate the two fields into one, according to the specified format. This approach allows for the creation of a new dimension that can be used across various reports and analyses within the model, maintaining the integrity of the original dimensions.

References:

SAP Analytics Cloud Help Documentation: Creating Calculated Dimensions

SAP Analytics Cloud User Guide: Data Modeling Best Practices