Personally, I would say:
1) "fantastic" = brilliant, really good
e.g. "your cooking is fantastic!" or "Fantastic news! She's had a baby boy!"
Often it is used sarcastically (especially in Britain) e.g. "It's raining again. Fantastic."
I would NOT say "fantastical" in these sentences.
2) "fantastical" = amazing, but also relating to fantasy, not real
e.g. "She has some fantastical idea to climb Mount Everest" or "He painted a fantastical scene with monsters and dragons"
In 2), you could MAYBE say "fantastic" in these sentences, with the same meaning..... but, I personally think this is a bit old-fashioned and less common.
It could also be confusing e.g. "She has some fantastic idea to climb Mount Everest" - do we mean it is a brilliant idea, or an unreal crazy idea? (It would perhaps depend on the tone of voice, who is speaking, etc...)
I think the history of the words is the same, but in practice they are used differently nowadays. (Like the person on Stackexchange, I think dictionary.com is not up-to-date)