You’ll probably want to check out this document:
http://codex.ww.wp.xz.cn/Widgetizing_Themes#Create_New_Widget_Area_Using_Custom_Function
If you only amended the sidebar-home.php, then the widget area should still be available in the widgets dashboard.
If you want to make another one with 2 across, you could also solve that by adding some specific css.
So: if this is just a requirement for one site, I’d go for the css-solution. If you want to deploy it for multiple sites, consider adopting the theme further.
I’m having trouble getting this to work. I’ve never done anything with Theme Specific Hooks before, does this theme use them?
I have only amended the sidebar-home.php in the child theme, but no new widgets appeared in the Dashboard.
And yes it is only for this site, not multiple sites.