You can hook up multiple subs to one amp. Also, if you are using multiple amps with your setup does one of them have an output so you can daisy chain amps??
Is your alternator even going to support that much wattage???
About hooking the amps together. If you go to ED's website it mentions that those amps are stackable and can then put out more power. If you allready have the amps and subs at your house, check out the manual it should give specs on it. You should not have to mess with using a Y cable at all.