It is well known that the grammars of Korean and Japanese are very similar. So learning one from the other to an extent amounts to learning vocabulary (e.g. particles) rather than grammar proper. Are there resources (in English) comparing the two, for instance suitable for someone learning the two languages? I found a table showing particles in the two languages for instance, but nothing extensive.
Japanese and Korean is similar, but somewhat much different. It is like German and English, or French and English. There are a lot of similarity in Japanese and Korean grammar(like SOV order) but (I don't know Japanese well so I can't tell, but) there's many different(like adjective/determiner in Korean, いadjective/なadjective in Japanese).
But in vocabulary, many words are similar, because of using sino-Korean/sino-Japanese. You can think it is like there are many similar vocabulary in European language because of Latin.
You can compare some differences or similarities, but you can't learn both languages at once. Languages are very huge stuff, with vocabulary, grammar, character and even cultures.
Even Korean and Japanese have similar words and grammar, they have different cultures. So, modern Korean and Japanese are not similar at all. (I mean, mostly teenagers and online languages)