I've just installed Linux Mint 12 Lisa and would like to extend the bottom taskbar to both monitors.
Lisa uses a fork of Gnome Shell (Gnome 3) called Cinnamon
How can I do this?
Cinnamon 1.8 does not yet support extending or duplicating the taskbar to multiple monitors. There is an open issue on github with some support, but no progress yet as far as I am aware.
For extended taskbar support on Linux, see The State of Linux Dual Monitor Support.