Hi guys

I've got an aftermarket, mechanical water temp gauge and installing it on my xd. I want to put it in the radiator & keep the stock one as is. Do you think this will mater, and where in the radiator should i place the sender???

Does it have a screw in fitting?

You will probably find a spot in the manifold or use an adaptor in one of the T pieces to screw into.

If you are going to put it into the radiator you need to put it in the "Hot" side. That is the side with the top hose on it. If its a screw type connection then you will need to connect it into an existing hole in the manifold or use an adaptor. Or you may make your own into the thermostat housing.
Running both gauges will be fine, you will be able to check on how accurate the factory one is.