yes, just add two fields to the sort records script step or dialog box (ctrl/cmd + S)
If you add the subsummary field first, then the records inside of the subsummary will be sorted by any field(s) after that.
In a subsummary by Company Name would return contacts, grouped by company, sorted by LastFirst name order.
To add to Mike's answer, there are two different script steps that sort records. One only allows you to sort on a specified field, it replicates choosing a sort order from a column header in table view. The other script step, Sort Records replicates sorting as it works from the Records menu. It uses the same dialog for setting up your sort order and thus you can sort on more than one field.