Are the projects both in the same portfolio and program? Is there data points that connect the projects together?
If so you can probably do it if there's some sort of trigger or watch on project a when the data is filled in for project A then go search for the other projects and fill in the data points.
We have captured cross project metadata at the program level and then pushed down to the projects. Fill it out once on the program level and it pushes to all the projects below.