Under the virtual AP profile that includes the SSID you use, you can add both VLAN's. I am assuming that you meant 2 different VLANs. If not, then you will have to have some other device be the default gateway rather than the controller. Aruba controller's dont support secondary addressing.
If you do have 2 VLANs, add both to the virtual AP profile. You cant control which of them the user is placed into, but you should get a roughly 50/50 split. You can then remove the first VLAN and only affect 50% of your users.
When you add the VLAN to the VAP, you will cause the users who should switch to the secondary VLAN to bounce, so you should probably do this during down-time (which may mean you don't even need to keep the original VLAN anyway).