Does your teacher object to your studying a different style of aikido, or to your studying with anyone from a different organization? For example, a teacher of Nihon Goshin aikido or Korindo aikido might be concerned about protecting that style from being corrupted by the influence of Ueshiba aikido, which is quite different, especially if you are a teacher in his style.
On the other hand, after 15 years of practice you should be able to understand how to use any newfound knowledge.
I would say that my Sensei is loyal to organization X and only X. Other organizations are not talked about or referenced. I understand the need to follow your teacher for a while especially when you are a beginner as it may be confusing to learn at that stage.