Installation of SQL Server 2014 and SQL Server 2012 on the same system I have a system with a SQL Server 2012 Developer edition installed.

Can I install SQL Server 2014 (6 months trial) and SQL Server 2012 on the same system?

I do not want to overwrite the pre-existing SQL Server 2012
Yes you can! Your first SQL Sever (2012) will most likely be the default instance, any subsequent installation of SQL Server (any version) must be a named instance. As long as your machine has the resources, you can install multiple instances of SQL Server.