Windows 2008 backup domain controller

Hello I have 2 domain controllers in the same domain called solar.local
In AD users and computers i see both DCs in domain controller ou. When i create object in one dc it appears in another as well.
I wanted to check whether dc2 would work as failover if i turn off dc1. So i did. Then i wanted to add a host to domain. But i couldnt. I had to power on dc1 to do so.
So whats a point of dc2 as backup if i cant join host to a domain.
Or therere something i need to do in order to make it happen
pls help