How do I sync instance A to instance B without losing any of the data already on instance B? Like I said in a comment below, I'm no DBA and I'm still in a learning process.

Here's my scenario :

I have an Oracle server running a couple schemas (let' call it Source). What I would like to do is send a 1:1 copy of every objects to another Oracle server (let's call it Target) where there's already some objects that I need to preserve.

Ideally, I don't want to shutdown the Source server.

Also, I need to keep the already existing schemas of the Target server. So the new objects from Source need to co-exist on the Target server with the already existing objects. Maybe by using 2 instances that could be accomplish? I don't know, I'm just suggesting.

What could be the solutions to achieve that?
Golden Gate is answer to these types of problem. Oracle bought it. Therefore it is a Oracle Golden Gate right now. 

Look below image which is taken from Oracle Golden Gate Admin Guide
 http://download.oracle.com/docs/cd/E18101_01/doc.1111/e17341.pdf

Also you can look to following article: 

http://www.dbasupport.com/oracle/ora11g/Oracle-Replication-Streams-vs-GoldenGate.shtml



Thanks Tevo D for pointing out licensing issue. Actually golden gate may be a expensive solution depending on your circumstances, i.e your company's standing with Oracle, what is your cost of downtime etc.