Thanks for the quick response! The action interface looks indeed interesting. I am not sure whether it is possible two have to source datasets. All the examples and also the action:python example has only one data set as input.
The action summation sounds as it could be the right thing, but it is not clear what it sums up. It is mentioned in the documentation as pre-implemented action, but it does not appear in the XML reference. Is this a mistake or is it not part of preCICE anymore?
Edit: Regarding summing up data from multiple input datasets. Could I have a chain of actions that write the source data into the target data after each other?
Edit 2: I also realized that I run into the problem that I technically want to map from two meshes onto one which was not well defined according to the uekerman’s post. I am not sure If I can easily workaround this using actions. I gave it a quick try, but I failed so far. 