As an adjective the terms are quite different:
Just - based on what is morally right and fair
Only - single or solitary
alone deserving consideration
As an adverb:
Just - exactly
Only - solely
no longer ago than
with the unfortunate result that
So, while they can mean the same thing at times, given context, they could also be drastically different things.
Also, you can see that just can mean nearly opposite tings when used, depending on what word it modifies.