all standard stuff really, just check the fetchmail manpage. you can put as many local and remote accounts as you wish....
poll pop.provider.net proto pop3
user "jsmith", with password secret1, is "jsmith" here;
poll other.provider.net proto pop2:
user "John.Smith", with password "My^Hat", is "John.Smith" here;
i'd suggest it's not really a good plan though, why not just use procmail to put all mail from one account into one folder, others in another. that way you only need one local user account.