How do I add values on nodes in two files

Hello everyone, I have solved two XDMFFile files through fenics, and the two-dimensional grids in these two files are the same. I want to merge the two files into one, that is, the sum of the two. How can I achieve this idea?
Thank you so much!

Since the grids are the same, I presume by merging, you mean you want to merge the data arrays. If the two meshes are exactly the same, then you can use Append Attributes. Open both files in ParaView. Then select both of them in the Pipeline Browser and create the Append Attributes filter and hit Apply. The output will include data array from both inputs.

Thank you for your answer, this can combine the two files into one, the next question is, I want to sum each node value of the two separately, so as to get the cloud image and color bar after the addition, how do I achieve?

Try applying the Calculator filter next. In the expression, you can specify the variable names to add e.g. var1 + var2. The sum per element will be saved in an array named Result. You can change the name of the result array on the Properties panel for the Calculator filter.