Example of how to map a double-tap of a modifier (⇧ / ^ / ⌥ / ⌘) to another keyboard shortcut. In this example ⇧^⌥⌘0

KeyRemap4MacBook Double Modifier

<item>
    <name>Double Control</name>
    <appendix>Doubletap ^ to ⇧^⌥⌘0</appendix>
    <identifier>private.smithjw.DoubleControl</identifier>
    <autogen>
        --DoublePressModifier--
        KeyCode::CONTROL_L,
        KeyCode::CONTROL_L,
        KeyCode::KEY_0, ModifierFlag::SHIFT_L | ModifierFlag::CONTROL_L | ModifierFlag::OPTION_L | ModifierFlag::COMMAND_L
    </autogen>
</item>