Simple solution is configuring those 2 hdds as Raid 0, that will make your cache very fast overall.
Other option is simply add new hard drive, mount it and in squid config file, specify another line defining new cache location. That way new cache as well as old, both will be used.
eg.
cache_dir ufs /cache-location1 3000 16 256
cache_dir ufs /cache-location2 8000 16 256