Writing everything twice for the speculative success of a brand new Reddit clone doesn't qualify as "deliver as fast as possible" to me.
Edit: For clarity, I mean there would be significant overlap in areas like data retrieval, sessions/auth, etc.
A Reddit like app would have read/write to and from the browser and to and from the database for many data structures. Users, admins, moderators, topic areas, topics, threads, users, etc. And different views depending on context. You see more of your own profile, less of others...and similar for moderated topic areas and so forth. So, 2 platforms means duplication of some of this data access, update, marshalling, acls, sessions, etc.
Assuming the node.js part is for the "real time posts", via websockets, it would need most of the above. Then, assuming php is handling the rest, including profile edit, rendering everything but the posts, etc...it also needs most of the above.
