This question is about a similar situation as described in How do I coordinate the process of pursuing multiple job opportunities at the same time?, but that question focuses on the situation after you got interviewed.
However, what about the situation where you already know that you will not be able to give a prompt decision even before the interviews start, especially if you expect that the time span will be longer than the 3 weeks mentioned in this answer?
For example given the following situation: You got contacted by a recruiter and he wants to schedule for an interview with you. However, while you are very interested in the job, you already know that you will not be able to accept a potential job offer in the next two months. I could imagine multiple reasons for this, but I do not want to bias the question.
Is it better to decline the interview invitation now and try to contact the recruiter/company again later (for the same or other similar jobs) or schedule an interview and then potentially delay the acceptance?
Edit: To answer the clarification requests: I live in Germany, but answers about other regions would be interesting, too. I specifically talk about the situation where you are not "able to make a decision on whether or not to accept a job offer for two months".