About keyboard layouts switching.

[Sand Wraith]

I have two layouts on my system: English and Russian, and alt+shift combination for switching, xorg.conf:
...
option "XkbLayout" "us,ru"
option "XkbOptions" "grp:alt_shift_toggle,grp_led:scroll"
...

how can i switch them with left shift and right shift (left shift enable "ru" and left shift for "us")?