The post mechanism uses a form with a target so you would need a single action. You could either add a checkbox ("create another") instead of a second button or otherwise use a hidden checkbox, set its value from the "save and create" button in Javascript and then submit the form from teh same javascript.
In the action, just check for the value of the checkbox in the post() data and redirect depending on what it is.
it’s called tabular input. there is an extension to do it in a grid view (Tabular Form WIdget). I don’t know if it will work in your case (gridview) if not then you can create your own.
for just a form
foreach in actionCreate to loop through each model instance and save into db.
do a foreach in actionUpdate to retrieve all models to update from db.
Yii2 Wiki Article
Another Extension
view page you will need js to copy form fields and duplicate the add item section
JS Plugin to copy form fields and demo
Then save would just save all of them and the "add more" button would just duplicate the fields for more input.
I don’t know if any of the article or plugins are good but they should defiantly lead you to the right answer.