I think you are looking for “append attributes” filter. If both of your datasets have identical grids and orientation, that should work. I have never tried it with unstructured grids, I think pvd can have many different types of grids. But I made an example with the wavelet source.
I loaded wavelet and made a 21x21x1 grid, saved it as a.pvd. Then ran calculator filter to create Result (added 100) , “pass arrays” filter to just get just Result, then saved b.pvd with only Result field. Reset ParaView. Loaded both a.pvd and b.pvd, highlighted them both in the pipeline browser and ran the Append Attributes Filter. This returned a single dataset with both RTData and Result, then I warped by RTData and colored by Result to show they are both there.
Agreed, Resample with Dataset is useful here, you can also resample both data sets to image data, if they are not already, and then run append attributes. Lots of ways to skin the cat depending on what you are starting with. … And what you want.