If these visits only occur occasionally, it may be best to keep these dogs separated so they aren't so tense and to prevent the dog from living in the home from rehearsing the protective behavior. You can erect a baby get or exercise pen and give each dog its space so there is nothing to fight over. Make sure they have some space in between so that they aren't face-to-face directly. It is best to avoid giving the visiting dog too much attention as the resident dog may feel more likely to resource guard. If the visiting dog needs to be given attention while the dogs are in their separated areas, best to have somebody feed the resident dog some treats/kibble so that some positive associations are created. You want the resident dog to think "how cool, every time my owner gives resident dog attention, I get some awesome treats!" Once the attention is no longer given, the resident dogs no longer get treats.
If these visits occur frequently, it may be worthy to implement behavior modification with the help of a professional using desensitization and counterconditioning based techniques.