i've got 2 amps for my subs,but only one pre-out from HU of course.i know i should be using amp's line out to connect the other one,but it is unable to send unfiltered signal from it,so i'm using Y-splitter to feed both of them and it seems to be fine(is it?).But when i think about it,are these 4volts getting to BOTH amps line-in or are they divided,so i'm getting only 2volts each? Nobody could answer me this question,so i'm asking you guys,thanks.
Your h/u is seeing half the impedance, but voltage should remain the same(4v to each). A decent h/u will simply provide twice the current w/o a significant voltage drop.
Tell him to prove it. In fact ask him to stick 2 pins in the wall outlet and touch them. If he survives that, then politely enquire about which pin gave more shock. The correct answer should be "equal".