In this case, notice that Fenix doesn't list 2xAA battery tube as an accessory, and shows voltage range "Input voltage: 0.9V~1.7V". While L2S shows "Input voltage: 0.9V~3.3V" and accessory 1xAA battery tube.
So they appear to be different heads, and if you want the option of using 1 or 2 AA, then buy L2S.