Some users have more than one server to which they connect that use PAP. Provided that your user name is different on each machine to which you want to connect, this is not a problem.
However, many users have the same user name on two (or more - even all) systems to which they connect. This then presents a problem in correctly selecting the appropriate line from /etc/ppp/pap-secrets.
As you might expect, PPP provides a mechanism for overcoming this. PPP allows you to set an 'assumed name' for the remote (server) end of the connection using the remotename option to pppd.
Let us suppose that you connect to two PPP servers using the username fred. You set up your /etc/ppp/pap-secrets something like
fred pppserver1 barney fred pppserver2 wilma
Now, to set connect to pppserver1 you would use name fred remotename pppserver1 in your ppp-options and for pppserver2 name fred remotename pppserver2.
As you can select the ppp options file to use with pppd using the file filename option, you can set up a script to connect to each of your PPP servers, correctly picking the options file to use and hence selecting the right remotename option.