It's OK grammatically, but it doesn't mean what you think it does. If you say that someone is 'supposed to xx', this means that it's something that they need to do or are scheduled to do in some way. For example, you might say "I'm supposed to hand in my essay tomorrow but I won't be able to do it. I'll ask my teacher for some more time" or "I'm not supposed to be here at work today, but I'm covering for a colleague who's sick". Do you mean "I don't suppose that SG and JB will ever get back together"? Or, if you want to make them the subject, you could say something like "SG and JB aren't likely to ever get back together".