Twin ALCO's are provided for -
1) More Horse power - 2 WDM3A locos provide 6200HP while 1 WDP4D loco provides 4500HP. This results in better acceleration at higher speeds. The train can achieve MPS faster.
2) Better tractive effort - Using 2 ALCos provides more pulling force than using a single WDP4D. So, the rake will accelerate faster at slower speeds, making it easy to accelerate after halts. Also, it provides better grip on the tracks on gradients and wet conditions.
No,...
more... no extra power is required from the locos for the AC. Twin locos are used as they provide better performance than a single loco.
WDP4D's are designed for 135kmph while ALCOs are designed for 120kmph. But the Pune Shatabdi uses WDP4D's as it has a lighter rake and a WDP4D is sufficient for the rake.