Can Squid benefit from SMP systems?
Squid is a single process application and can not make use of SMP. If you want to make Squid benefit from a SMP system you will need to run multiple instances of Squid and find a way to distribute your users on the different Squid instances just as if you had multiple Squid boxes. Having two CPUs is indeed nice for running other CPU intensive tasks on the same server as the proxy, such as if you have a lot of logs and need to run various statistics collections during peak hours. The authentication and group helpers barely use any CPU and does not benefit from dual-CPU configuration.
