To senior researchers, how would you view a student you wanted to work with, but who could not? 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?
Personally, I am much happier when somebody tells me "I can't do this because XXX" than when they disappear on me.  If they tell me they can't, then I can adjust my plans and act accordingly; if they don't tell me, then they leave me hanging and I find it quite frustrating.

I strongly advise you to make a clear decision about whether you are going to work on the project right now, and then go and clearly tell the person, through a direct video/phone link rather via email if possible.  If the decision is no, because your current supervisor is pushing you to focus on something else, then that will be quite understandable and will make no prejudice against you for most people---it is a quite typical difficulty for many people in graduate school.

As for whether they will be willing to recommend you as a postdoc: personally, if I like and respect a student, that's not going to be changed if external circumstances disrupt our work together.  If would not be as strong, of course, as if we had done more good work together that I could talk about more, but it would still be positive.  Others, of course, may vary in their opinions, and you can only find out from your associate by asking.