You cannot use 1 dial up by 2 people at the same time on 1 PC.
What you can do is as administrator you can allow guest account to use dial up.
Click on help & support type guest account in search. XP will bring glossary & how to turn on/off guest account. read the glossary & follow out the guidelines but it is only the administrator who can make changes to to what your guest accounts are allowed to do. My grandchildren use my PC when they visit on one guest account only & my admin account is locked by my password. So they cannot stuff up the programs accidently or watch restricted sites. They do visit music sites & children sites & download their programs for themselves.