Currently there is no function to send a (separate) reminder/mail notification to a user AND his/her boss in case, that an invoice is untreated within a certain time frame standalone.
There is just a function to push forward this invoice to the user's superior (manager) combined with a mail notification on that. Basically this function is good but it might be unwished or unnecessary to do so, in particular if this process would lead that all untreated invoices are ending up in top management's inbox for action.
Better would it be to have a standalone reminder to the approver and his/her boss which asks the user/s to take an action given that the invoice is overdue/untreated since x days. That kind of reminder will motivate user more to take action, given that also his/her manager is alerted by that.