It'll show two batteries, for the standard and the dock battery. If the device doesn't have the secondary battery (if it's not the Asus Eee Pad Transformer) it will hide the second battery

The widget doesn't run a service or anything else.. It will only register a broadcast receiver for the battery status as long as you have a widget on your screen. If you don't have one, it won't be doing anything at all (hence very battery friendly)

You can install it from the android play store here: https://play.google.com/store/apps/details?id=org.flexlabs.widgets.dualbattery

Note: On the current release of Honeycomb, it is possible that the widget will not immediately appear in the widget list after installation. To refresh the widget list, you should either restart your device, or restart the launcher app (Settings -> Applications -> Manage applications -> All -> Launcher -> Force stop)

Recent changes can be found in CHANGELOG.md

Note: To compile these sources you will have to pull down the linked submodules of this git repository. To do this run these git commands after you clone:

$ git submodule init
$ git submodule update