What Buckeye said. Can you describe the other sub in more detail? Is it a raw driver or a passive or powered sub? Line in is exactly what it says; a line level INPUT, so you aren't going to get anything OUT of it. If this is a raw driver, a line level out will not do anything either without an amplifier for that signal. No sub that I've seen will output a powered signal to drive something else, so trying to use the X-sub's amp to power something else simultaneously isn't going to work.