Board Thread:Warhammer 40k General Discussion/@comment-5846164-20130527133906/@comment-3258762-20130613181750

The Second Heart is an emergency organ, it only kicks in when the primary heart stops to function. It gives the Astartes body time to re-start the heart, and then they work together for a short while until the second heart goes dormant again. It is also a very small heart, it is enough to keep you alive and moving, but not fight effectively.

It has only one purpose: To keep you alive long enough for your real heart to start working again, but if your secondary heart goes off during battle, you're basically dead anyway.