Hello,
In ACS internal database (and in most common databases in general) the user-to-password mapping is one-to-one.
So as long as the same user is configured to access both Juniper and Cisco devices, the same password must be used too.
A way I could think to work this around would be to create the same user (with different passwords) on two different databases (like the internal one and AD, for example).
Then, through a Network Access Profile (NAP) on ACS, you can enforce to check the user's credentials against a specific database for Juniper devices and against the other database for Cisco devices.
Regards,
Fede
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