Older Continuum access controllers (CX9900, CX9702, ACX700, ACX781) can only store one card credential per user in their local databases as opposed to the newer CX9680, ACX5720/40 that store two credentials per user.
What is the behavior of said older Continuum access controllers when the system is configured for dual credentials?
To allow older controllers to validate the second credential enable remote validation in the controller by setting its Access Server system variable to the ACCNetId of the primary access server (for consistency, in actuality it can be set to any number in the range 191-254)
When configured for dual credentials CyberStation will not distribute credential2 to older controllers that can only store one credential, however, when the second credential is presented at a door configured at an older controller the controller will send a remote validation request to the primary/secondary access server and access will be granted or denied appropriately.
Note also that changing the Allowed Credentials at the CyberStation has no effect on older controllers that have no concept of the Allowed Credentials functionality, if for example the Allowed Credentials is changed from Credentials 1 and 2 to only credential 2, controllers that support the new functionality will stop granting access to credential 1 but older controllers will continue to grant access to credential 1 until reloaded.