Angular Material Set Multiple Columns in One Row

Impossible is nothing ;)

You can defined two mat-header-row

<table mat-table>
  ...
  <tr mat-header-row *matHeaderRowDef="['1','2','3']"></tr>
  <tr mat-header-row *matHeaderRowDef="displayedColumns"></tr>
  <tr mat-row *matRowDef="let row; columns: displayedColumns;"></tr>
</table>

Then you can defined the "mat-column-def". See that the fisrt you use attr.rowspan

<ng-container matColumnDef="1">
    <th mat-header-cell *matHeaderCellDef [attr.rowspan]="2">No.</th>
  </ng-container>
  <ng-container matColumnDef="2">
    <th mat-header-cell *matHeaderCellDef [attr.rowspan]="2">Name</th>
  </ng-container>
  <ng-container matColumnDef="3">
    <th mat-header-cell *matHeaderCellDef [attr.colspan]="2" class="center">
      Values
    </th>
  </ng-container>
  

Finally you need add display none to the th not divided

  <ng-container matColumnDef="position">
    <th mat-header-cell *matHeaderCellDef [style.display]="'none'">No.</th>
    <td mat-cell *matCellDef="let element">{{element.position}}</td>
  </ng-container>

  <!-- Name Column -->
  <ng-container matColumnDef="name">
    <th mat-header-cell *matHeaderCellDef [style.display]="'none'">Name</th>
    <td mat-cell *matCellDef="let element">{{element.name}}</td>
  </ng-container>

And create a class to center the colspan

.mdc-data-table__header-cell.center
{
  text-align: center;
}

A example in stackblitz