Re: Canon 3D?
 

The T2i's will sync fairly well with a Y-cable on the shutter release - however - the cameras internally poll the shutter release port for the switch closure. This results in some offset, depending on the polling cycle of each camera. Using the external battery to fire-up the cameras will increase your chances of achieving good sync.

Another note on using mechanical switches for powering up the cameras and triggering the shutters... mechanical switches are prone to bounce which could possibly introduce some variance between cameras. Because of this, "purists" use solid-state switches for this purpose. I remain unconvinced that this matters and have achieved perfectly acceptable results with plain old mechanical switches. Your milage may vary.

Finally, most camera manuals warn that you NOT leave the camera power switches in the on position when introducing external power. This is because a surge could fry the camera. I've never had a problem with battery or A/C regulated power sources with Canon or Panasonic cameras, nor have I ever heard of anyone else having a problem. Again, your milage may vary.