I can confirm this with my Thinkpad z60m using an ultrabay advanced battery as the second one.
If both battries are inserted during startup gnome-power-manager detects both and details are shown in the info dialog.
If the ultrabay battery runs out of power gnome-power-manger happily suspends my thinkpad ignoring the main battery.
If I insert the second battery while gnome-power-manager is running it will not be detected.
I can confirm this with my Thinkpad z60m using an ultrabay advanced battery as the second one.
If both battries are inserted during startup gnome-power-manager detects both and details are shown in the info dialog.
If the ultrabay battery runs out of power gnome-power-manger happily suspends my thinkpad ignoring the main battery.
If I insert the second battery while gnome-power-manager is running it will not be detected.