But ... since Derrick (starting with 50 BTC) makes 2 transactions, why there is only one change?
Is this wrong?
Derrick sends 25 BTC to Gloria - and will get 25 BTC change.
Derrick sends 15 BTC to Rustie - and will get 10 BTC change.
Derrick could either make a single transaction spending the 50 BTC UTXO with 3 outputs (25 to Gloria, 15 to Rustie, 10 as change), or Derrick could make two transactions with 2 outputs each (first transaction spending the 50 BTC UTXO with 25 to Gloria and 25 as change, followed by a second transaction spending the 25 BTC change UTXO with 15 to Rustie and 10 as change). The final outcome is the same regardless - Gloria has one UTXO with 25 BTC, Rustie has one UTXO with 15 BTC, and Derrick has one UTXO with 10 BTC (change).
As BHC has pointed out above, making two transactions instead of one will just cost a little more in fees.