Sorry I don't see any option to enable sorting on 2 columns ( first fixed one on group column i.e., company, and second on any other column ) with single: true. It works with single: false only.
Secondly it's not feasible to sort records in one group and ignore records in any other group. All the records are sorted.
I'm still looking for any available workaround to meet your requirements and would let you know as soon as I found one.