Maybe creating a non persistant entity with the data of both entities is an option here?
Adding upon Maartens suggestion, I would say that you should create a non-persistable object in a data view with a microflow as data source. In this microflow you can use the order object, that's probably available on the page anyway, as an input parameter. In the NP-object you can keep track of all the variables you want to display in your progress indicator (total amount of parts, amount of good parts etc.). In the data source microflow you can retrieve the parts related to the order and set the correct values in the NP-object. Lastly you need to make sure the NP-object is used as a return variable at the end of microflow. Hopefully this helps!
Thank you guys, that worked