My recent experience with the Y cable and 2 MagicShine lightheads is that the run time is not 1/2 what it would be for 1 lighthead. I didn't quite understand why, but now I think I do. Thanks moggy82. I think you're right. The gauge of the cables is leading to a drop in voltage that the lightheads are detecting as reduced battery voltage. I leave home with a fully charged 4cell pack, and the indicator on 1 light head turns red after about 35 minutes. The other one will usually go another 5-10 minutes before doing the same. They continue to have full brightness for at least another 20-30 minutes, but it's unnerving nonetheless.