$WOOCS->is_multiple_allowed

This is not a function - this is an important variable in the plugin main class WOOCS, and it's always shows is 'Is multiple allowed' options enabled or not. Predefined in the plugin settings.

global $WOOCS;
if($WOOCS->is_multiple_allowed){
    //do smth here if 'Is multiple allowed' mode enabled
}

If you want let your customers pay with their selected currency on the front end of your site - you should set multiple mode to 'Yes'

Is multiple allowed option in the plugin settings

Shortcodes, functions, actions and filters reference for FOX WooCommerce Currency Switcher