I think the closest thing you will be able to do have Set 8 to be a placeholder for what is displayed on your WXHB.

Assign the skills you want to set 6 and 7, but each one needs to have a macro.

Set 6 needs a macro that says
Code:
/crosshotbar copy JOB 7 JOB 8
Set 7 needs a macro that says
Code:
/crosshotbar copy JOB 6 JOB 8
You won't get the desired behavior you described, but you'd get something close.