Once the users entered in the database of ACS, no way to seperate them based on RADIUS or TACACS protocol.
However you can have one user always authenticted using RADIUS and not using TACACS using "NAS Filtering" or NAR . NAS filtering can be used to limit authentication of a user on a per-NAS basis. So with that the user will be authenticated from RADIUS speaking NAS only and not via TACACS speaking NAS. (if its configured that way).
In other words you can dedicte certain users to be authenticated by certain NAS only.
Here is the way to do that
http://www.cisco.com/univercd/cc/td/doc/product/access/acs_soft/csacs4nt/acs31/acsuser/u.htm#94180