I am a Ph.D. student and met this senior scientist in a local conference, let's call him Dr. XX. (He is working in a research institute like Bell Lab, not exactly in academia.) We took some interest in each other's work and my supervisor allowed me to work with him. We (me, Dr. XX and my supervisor) just got our paper accepted in a journal as well. I liked working with him and learnt from him.
He also raised some excellent idea about how to extend the work to another novel domain and has been pushing me to come up with some literature study and run some experiment etc. Unfortunately, my Ph.D. supervisor (who is the boss I answer to) is not exactly enthusiastic in that and pushing me in a different direction.
I have conveyed it to Dr. XX in somewhat evasive terms, telling I need some time to decide etc. but just realised it has been a few months. I am not sure how to deal with this. I love both of the topics for the later stage of my Ph.D. but how do I prioritise?
Also, I know it is a question of interpersonal dynamics, but if you were Dr. XX, how would you think of me for being evasive? For example, while applying for postdoc position, will you write a recommendation for me, with this somewhat awkward situation?