If im the account holder for two lines how can i check for the other lines outgoing/inbox txt messages?

Yeah, it sounds like you’re trying to spy on someone. That’s not cool. If you have beef with someone, talk to them. If you think they’re committing a crime, go to the police. Snooping around is only going to make it worse in the end, whatever horrible thing you feel has happened.

Why don’t you just look at your phone, if it’s your line?

You can’t do it without the actual phone unless your carrier provides detailed text info; mine does not.