Introducing new animals should always happen whit patien and consideration. Sometimes it takes a while before the animals are used to each other. You can sometimes see a small struggle when they meet and examinre each other. Keep a close eye on the animals and make sure the introducing dos not get out of control and transforms in a real fight.
Proposing the animals to each other, at first begins with just making sure the animals can see each other. From a distance they can begin to communicate with each other through body language. The best way to do this is by placing the terrariums next to each other, so they can meet and communicate from a save distance. When the animals are used to each other and interact well, you can go a step further.
Gently introduced them on neutral ground. If this goes well then repeat this a few times. If this goes goed as well, it's time to put the animnals together in their terrarium.
In some cases the introducing goes right the first time. It could also be, that the bearded dragon respond negative on each other, what can result in a fight. This can be notice by stress signals and body language of the bearded dragon In a normal introduction you will sometimes see that they rotate around each other. If this is too violently and the also trying to bite each other, this can be first signals of a fight starting.
Negative behavior while introducing, can be caused by territory motive or by fear (stress). Try to avoid negative encounters. So be patient when you introduce your animals to each other.
There is no guarantee that the couple is successful, some animals can simply not get along.