Cannot deactivate caps lock once turned on

[apotenza92]

I simply want both shift keys to activate and deactivate caps lock.

For some reason I'm finding this really difficult to achieve. Here's the simplest thing I've tried:

{:des "Both shifts to toggle caps lock"
         :rules [[[:right_shift :left_shift] :caps_lock]]}

This works to have caps lock turn on. But repeating the same keys will not turn it off!

Can anyone help? I must be missing something basic.