From a cursory look at the source code, it appears that the plugin is only meant to scroll one child at a time. It would take modification of the source code to do what you require. So to save us both a lot of work, have you looked at alternatives?
Update: Ok making the modification to scroll just 1/2 the view port window was much easier! I posted a demo for you here. But instead of making the modification permanent, I decided to make it an option - these are the settings I used with the new option "moveHalfViewPortWidth" set to true:
var jac2 = {
enableMouse : false,
childSizeFixed : false,
moveHalfViewPortWidth : true
};
// Setup galleries
$(".jac2").jac(jac2);
download the modified plugin from here.