Just tighten them down the ground face of the union will make the seal.
Do I need to put anything between two halves or do I just crank it down to seal it?
A couple of things...make sure the pipes are aligned when you tighten things up. And, especially if you overtighten, you may find you create a ridge on the sealing surfaces, so if you need to tak it apart, it becomes more difficult to seal unless you get it aligned perfectly.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013
I usually use pipe joint compound on the faces and the threads of the union nut to give some lubrication to the process.
Do to the fact that most fittings are made off planet, I also use a little pipe compound on the face, your choice.