Kalk doesn't supply any additional elements other than whatever impurities are in it. I see no real reason to run both unless your reactor can't keep up.
I would get as much from the CaRx as you can since it has more in it than just calcium and alk... it has all of the elements that were necessary to build the coral skeletons that you are now dissolving.
This assumes that you are using natural media. Man-made media only has in it what man put in there.