rockbird,
- Make sure that all your clients already trust both server certificates
- Make sure that all access points in an area put users on the same VLAN(s)
- On the Aruba Controller in the SSID profile, turn on "Advertise QBSS Load IE". Cisco access points have this on, and devices will prefer the Cisco access point over the Aruba access points, if the Aruba access points are not broadcasting this element. http://www.arubanetworks.com/techdocs/ArubaOS_64_Web_Help/Web_Help_Index.htm#ArubaFrameStyles/1CommandList/wlan_ssid_profile.htm?
You will have your most issues when VOIP devices are roaming between Cisco and Aruba access points. Try your best to do an entire building at one time, if you can, to minimize that. If you can, have your both of your infrastructures point at the same radius server, so that roaming will result in a radius resumption vs a complete reauthentication.
If you have not done so already, I want to say hire a professional who can ask the tough questions and make sure that you are doing everything you can to make this a success. There is plenty to keep in mind, but we do not have access to that infomation here.