trouble setting up dual layout keyboard

[Sébastien Barthélemy]

Hello,

I'm using X.Org 1.5.2 (the one shipped with ubuntu intrepid ibex) and
I would like to setup my keyboard in the following way:
 - use two layouts: US and french
 - switch between the layouts by pressing both alt keys together
 - use the caps lock key as compose

I tried this command:
$ setxkbmap -model evdev -layout "fr,us" -option "grp:alts_toggle"
-option "compose:caps"

Which results in
$ setxkbmap -print
xkb_keymap {
	xkb_keycodes  { include "evdev+aliases(azerty)"	};
	xkb_types     { include "complete"	};
	xkb_compat    { include "complete"	};
	xkb_symbols   { include
pc+fr+us:2+inet(evdev)+level3(ralt_switch_for_alts_toggle):1+level3(ralt_switch_for_alts_toggle):2+group(alts_toggle)+compose(caps)"	};
	xkb_geometry  { include "pc(pc104)"	};

However, it does not work perfectly:
* Pressing both alt keys switch from US to french but does not switch
back to french
* In the US layout caps lock key behaves as caps lock (bad). In the
french layout, the caps lock key behaves simultaneously as compose and
caps lock

I'm quite lost, and the doc I found [1] was not much helpful. I cannot
believe I'm the only one with such settings. Any idea why it does not
work ?

[1] http://www.x.org/wiki/XKB

-- 
Thank you in advance,
Sébastien Barthélemy