To boot a Mac using another Mac, you first require shutting down both Macs. Get a 6 pin to 6 pin firewire cable and plug-in the power brick in the Mac without default. Then, link both Macs through the firewire cable and make sure no other firewire peripherals are connected. Start up the working Mac while holding the T key. A firewire icon should show up on the display. Start up the other Mac while holding the Option key. The hard drive of the working Mac should show on the other Mac as a start up disk for you to choose. From here, then you can boot the machine.