I would say "The tension is mounting." OR As Sarah suggested, "The tension is rising." OR "The tension is climbing." All answers I just gave are correct and acceptable You could use "tension mounts" like this.. As the tension mounts, more people are focusing on the issue. OR As the tension mounted, my friend got nervous and farted. Everyone heard it. It was so funny!.

We don't really use either phrase all that often, instead I think it's more common to say "The tension is rising" instead of "mounting"

The second one.... Tension is getting /it is a continous action therefore, you need the ing form

Sarah correct, its not commonly used, however I would probably say Tensions are mounting