A gas turbine engine system (210; 310; 410) comprising: a first sub-system (212; 312; 412) having a first gas turbine engine and first and second electrical generators (218, 220; 318, 320; 418, 420) arranged to be driven by first and second spools (214, 216; 314, 316; 414, 416) respectively of the first gas turbine engine, the first electrical generator having a maximum output power P m1, the first spool having a lower rotational speed than the second spool during operation of the first gas turbine engine; a second sub-system (322; 422) having a second gas turbine engine and third and fourth electrical generators (324, 326; 428, 430) arranged to be driven by third and fourth spools (324, 326; 424, 426) respectively of the second gas turbine engine, the third electrical generator having a maximum output power P m1, the third spool of the second gas turbine engine having a lower rotational speed than the fourth spool thereof during operation of the second gas turbine engine; and a controller (232; 332; 432) arranged to provide first and second control signals to the first and second electrical generators respectively to control the electrical power extracted therefrom and to provide third and fourth control signals to the third and fourth electrical generators respectively to control the electrical power extracted therefrom, characterized in that the controller is arranged to receive a demand signal corresponding to a total electrical power demand P to be met by the engine system and to provide the first to fourth control signals in response thereto such that the demand P is met by extraction of electrical powers xP, (1-x)P from the first and second sub-systems respectively, wherein 0.5 < x < 1, and wherein if P ≤ P m1 /x, electrical powers xP, (1-x)P are extracted from the first and third electrical generators respectively. a first sub-system (212; 312; 412) having a first gas turbine engine and first and second electrical generators (218, 220; 318, 320; 418, 420) arranged to be driven by first and second spools (214, 216; 314, 316; 414, 416) respectively of the first gas turbine engine, the first electrical generator having a maximum output power P m1, the first spool having a lower rotational speed than the second spool during operation of the first gas turbine engine; a second sub-system (322; 422) having a second gas turbine engine and third and fourth electrical generators (324, 326; 428, 430) arranged to be driven by third and fourth spools (324, 326; 424, 426) respectively of the second gas turbine engine, the third electrical generator having a maximum output power P m1, the third spool of the second gas turbine engine having a lower rotational speed than the fourth spool thereof during operation of the second gas turbine engine; and a controller (232; 332; 432) arranged to provide first and second control signals to the first and second electrical generators respectively to control the electrical power extracted therefrom and to provide third and fourth control signals to the third and fourth electrical generators respectively to control the electrical power extracted therefrom, characterized in that the controller is arranged to receive a demand signal corresponding to a total electrical power demand P to be met by the engine system and to provide the first to fourth control signals in response thereto such that the demand P is met by extraction of electrical powers xP, (1-x)P from the first and second sub-systems respectively, wherein 0.5 < x < 1, and wherein if P ≤ P m1 /x, electrical powers xP, (1-x)P are extracted from the first and third electrical generators respectively.