In an Assembly, can I create a Plane that is always in the middle of two parallel component faces?
Use the equation tool. Assembly your two components with the "distance between", then add your datum plane offset from one of the parallel surfaces. Then go to the equation tool command and select "Top Level". In the first column select the dimensonal value distance for the datum plane and should look like this"D1@PLANE1"; then in the second column click in the space and select "measure" and pick the two surfaces that you want the datum to be centered between them and it should look like this "RD1@ANNOTATIONS" then add divided by 2 like this "/2".
Click ok and regenerate.