only you know what you really want measure your trunk then compare with the recommended cubic feet of each sub but a SP4 15 would be really nice since theres hardly anyone here with that type of set up lol
heres a link i hope it answers your question http://www.stevemeadedesigns.com/board/topic/131937-how-to-do-2-runs-of-0-gauge-wire/page__p__1854327__fromsearch__1#entry1854327