When I used Windows I could simply click the Eclipse icon and run a session, then click it again to run another. This way I could compare 2 different projects. With Ubuntu 15.04, is there a way to do this?
I take it you don't want to use tabs to compare the projects, anyway I'd recommend that if you use Eclipse from the Ubuntu repositories-which are very very outdated-to download an updated version from the official website https://www.eclipse.org/downloads/
For the Mars release there is a helpful Eclipse Installer or just download and extract the package you require.
To start open the file "eclipse" within the extracted folder. Opening this file again gives you another Eclipse instance. You can drag the icon into the Unity toolbar for easy access. Generally running more than one instance is only a problem if the same workspace is being used, because when saving modifications one instance would interfere with the other. So if you want to compare 2 different projects in 2 instances of Eclipse just have them in different workspaces. You could just quickly copy a workspace into the same directory, call it workspace2 and open the other instance using workspace2.
When playing around until you get the setup you need, a backup of your workspace(s) isn't a bad idea
It's a duplicate question. Btw, you may do as directed here: https://stackoverflow.com/questions/118243/open-multiple-eclipse-workspaces-on-the-mac
I did exactly same on Ubuntu 14.04LTS and it worked perfectly. Give it a try.