There are two ways to do this:
Option 1:
* You have your original ap-group that contains vap1.
* You define a second ap-group that contains vap1 and vap2. APs in this group will advertise two ESSID.
Option 2:
* You can add a second vap to the AP instance. This is effective if you are defining it for a one or a few APs. Any more, i would suggest option 1.
-michael