'Just' as an adverb has three meanings: 1. Very recently eg 'I've just heard the news'. 2. Exactly eg 'That's just what you need'. 3. Only eg 'He just speaks English.' The two words are only interchangeable in the 3rd situation. The two words are just interchangeable in the 3rd situation. I think that explains it!

don't use both of them

"Just" has too many meanings. Sometimes it is close to "only". You can learn that as well as "only" here: http://www.thefreedictionary.com/just In my opinion "only" is more specific, more strong.

I've already answered the same question. Do visit here: http://www.italki.com/question/254991