Thats because the two engines were mainly in-sync. I have some video in South Africa where a train was pulling pretty hard and you could hear the two distinctly. Same for N&W 1218(simple Articulated), and UP 3985. Most of the time you cannot hear the different engines.
Nice video and restoration. Whistle reminds me of South Africa Garratts or class 24 on the old George-Knysna line.
Also for those who get the spelling wrong you can see the builder plate with the correct Garratt spelling!!!
Cheers!
Greg