Many SQL Server users (starters and intermediate level users) have a common doubt about the possibilities of installing multiple SQL Server instances on a same physical server. The answer is a cautious yes; the reason for being cautious is, not being sure about the behavior of the application(s) using the previous version of SQL Server. Such a setup is called Side-by-Side installation. It is possible to have multiple Versions of SQL Server Instances on the same machine and also multiple editions of same version of SQL Server.
In the picture below you see that on the same machine, 4 SQL Server instances have been installed and 3 of them are SQL Server 2005 and the other being SQL Server 2008. Each one of the Instances is different from the other.
I used multi-server querying capabilities in SQL Server 2008 to find the information of all the servers in a single query. Coming to Side by Side Installations, it must be noted that this might be required when performing an upgrade on the same machine as the current one. This requires no additional hardware for setting up another system, but on the other hand, there is a risk factor in this, if the installation of the newer instances is unsuccessful and the system becomes unstable or the newer version of executables have known / unknown issues with respect to the working applications.
The decision to go for a Side by Side Installation or a new installation on a seperate system, varies. If it is being performed on a test server for some testing additional functionalities or features it would be a worth a try. Whereas performing this on a system which cannot have downtime, it is not worth a risk to take.