It is pretty simple:
Your datasource location (which you need to define in the Footer Rendering Item) will look like below:
query:./ancestor-or-self::*[@@templatename='Home Item Templatye Name']/Global Content/Footer Global Contetn|./Footer Local Content
You need to correct paths and Home page template name.
See this article: http://firebreaksice.com/sitecore-rendering-datasource-locations/
Basically there are a simple Sitecore queries in the paths above and the parameters can be different. You can read detailed about sitecore queries by the following link or google something like "Sitecore query" for building another queries:
http://sitecoreworld.blogspot.com.by/2014/09/querying-items-from-sitecore.html: