How to decline a postdoc interview if there is some possible future collaboration?
Don't go -- it's a waste of everyone's time if you go to an interview without any intention of taking the job. People get (rightfully) more upset about you wasting their time than you not wanting a job because you've already found one elsewhere.

Besides, if you will be required to collaborate with the person with whom you were to have your second interview, they (i) may have personal contacts with the research group where you accepted the job, (ii) may already know that you took the first job, and (iii) will need to be among your friends. Simply inform them that you took the first job (and not beat around the bush by saying "somewhere" or "think I am accepting" when you have already accepted). Offering to come anyway for a collaboration or a seminar is fine.