Time to straighten out the terminology. Back-to-back flights are a special subset of nested flights wherein the city pairs are the same and often interpreted by the airlines to indicate an attempt to circumvent minimum stay rules such as a Saturday night.

Other nested flights are perfectly "legal" everywhere.

Example of "legal" nested flights are AAA-BBB-AAA with BBB-CCC-BBB in the middle.
Example of "illegal" back-to-back nested flights are AAA-BBB-AAA with BBB-AAA-BBB in the middle.

Several years back my wife flew to PHL to pick up her elderly mother and bring ber back here for a week. The flights were back-to-back as in the second example above but the BBB-AAA-BBB was for 2 people. She was "caught" and threated with airline-divorce or somesuch and it took a little bit of dancing around to get them to accept there was no attempt to circumvent rules but rather to favilitate travel for the old woman.