Please find whether the other Company is using this particular tag line much before you have started to use, if that be so, they become the 'prior user' and the it follows that the 'prior user gets the priority in registration'.Also pl check whether they have already applied for trade mark registration with that tag line. If they have applied earlier,you can find their user date claim. If they are using that tag line for number of ytears before you have started to use, it is better you avoid using it and go for a new tag line. In that case you can even 'abandon'the earlier application and file a fresh application with your new tag line. It so happens sometimes that this Company might not have applied for registration and you may not be knowing their user date. In that case the TM Registry would register your mark. But the question is that if they come to know what they would do < Even if they do not have trade mark registration and if they fall under the 'prior user' classification, de3finitely they can go for action against you for using their tag line-under Common Law..They can go for 'passing off' action.
To avoid complications in future, it is better find whether this Company is the prior user and if that be so, it is better give using that tag line and go for another one.