Slightly different there, as with your one landline and 4 phones, you might have 4 phones but you can only use one line at a time, thereby only using one line's worth of the company's resources. If you have two cell phones, you can use them simultaneously, using twice the company's resources rather than just enough for one phone. We won't go into broadband sharing or call waiting or three way calling. There are "family plans" with shared minutes if that's the kind of deal you're looking for, but it doesn't sound fair at all to have two different active cellphones and pay for only one.
All the services are accessed through one fixed node, unlike the mobile phones. Hence two bills.
