- Can 2 servers in SharePoint farm have their own SQL server database? Or they must share database? because when I was installing second server in farm it said that servers must share config database.
- Can there be 2 central administration for 2 servers in farm? Or a farm can only have 1 central admin?
- What is the difference between SharePoint configuration database and content database?
Way before starting installation or implementation of SharePoint one needs to decide the topology they will follow as advised by Microsoft as best practices. I am actually very surprised by your questions specially when you are installing second server.
Anyway first thing you need to know is what a SharePoint Farm actually looks like,
Here's its definition is simplest words I could find
Back to your first question, each SharePoint farm must have one Farm Configuration database, so when you were installing or adding another server(which I assume was a WFE) you are either allowed to create a new server farm or simply join farm which is already there (Joing farm means you will use farm configuration database which is already there. Its common sense 1 farm = 1 farm configuration database).
One SharePoint farm will have just one Central Administration but you can access or host it on different servers if required, Normally it is installed on WFE server only.
This diagram is the best way to tell you the difference between different SharePoint databases,
Check this diagram by Microsoft out for more details.
I think you are really really confused with concept of Typologies in SharePoint and would recommend you spending some time on reading about it.
One thing I forgot to mention that a SharePoint farm can have multiple SQL Servers, Here's an MSDN article on how to add another Database server to farm.