The GBC is not a Windows Manager, hence the ability to only display the top most normal window plus one modal window.
In terms of missing something, you can use the Left hand sidebar to quickly toggle between two instances of the same application in the same GBC session. If you are trying to spot differences this may help (you will notice the flicker as values change), although probably not as good as having two instances of the same program side by side.
If you want to launch child programs in a new tab, there is the browserMultiPage presentation style
http://4js.com/online_documentation/fjs-fgl-manual-html/#fgl-topics/r_fgl_presentation_styles_userinterface_style_attributes.html which will do so without creating a new session. That was introduced mid-release to satisfy a customers pressing need and as such it does not solve everyones requirement, the typical request is to launch RUN in the current tab and RUN WITHOUT WAITING in a new tab. There will hopefully be an improvement in this area in the next major release. (it requires changes GAS and GBC so limited what we can do mid-release). Hence I tend to suggest using launchUrl front-call instead of RUN WITHOUT WAITING so a child launches in a new tab.
If launching from a web page, you can use the window.open method to launch in a new tab and control position and size e.g. window.open("
http://localhost:6394/ua/r/gwc-demo", "Window A", "width=800,height=600,top=0,left=0"); window.open("
http://localhost:6394/ua/r/gwc-demo", "Window B", "width=800,height=600,top=0,left=800"); will position two windows side by side
Hope that helps,
Reuben