Parking is where your domain is currently parked so obviously one domain can not be parked at two places same time. Sell page is nothing but an advertising post in different platforms to show that your domain is available for sale, so definitely you can put in many platforms until and unless you have any exclusive or binding contract with any registrar or agency.
Now when you add your domain they need verification and therefore you need to change your domain DNS to that particular site. After verification and successfully parking there either you can continue or change the DNS to another platform where you want to advertise your sale again.