linking different domains that I have installed wordpress on and would like to know how to link them

I have a few different domains that I have installed wordpress on and would like to know how to link them so that I only have to do one update, change etc. Should I have just installed on one domain and created sites instead? I had these domains as joomla installations originally, but have migrated most of them to wordpress and am trying to find the quickest and easiest way to maintain them.