Which replication technique to use?

I would like use Postgresql 9.3, i'm looking for a replication solution permitting this use cases.

there's 2 servers both can write data, so they must be synchronized with some frequency if the DSL is working well , if the DSL is cut, they had to write data and sync it later.

this case is very specific, if the second server goes offline, is there a manner to export delta messages to make synch but through files (these files could be exchanged through email)?