Multilingual WordPress site

Hi! Just found this forum and it is peeerfect for my problem. I would like people's opinion on a multilingual blog. I have two blogs that I want to seriously get back on. One is horse related and the other one is health related. As I know both of them is kind of too much, this would by now apply only for the health related, as the horse related one will be only for translations from English to Spanish of key info.

I am Spanish, but bilingual with English. The biggest audience is in English, so I want to write in English, but there is a huge lack of info about health in Spanish, so I feel the need to help on that too. And my question is, how can I manage to have the same blog but in both languages? What is the most efficient way and the least time consuming? Two separate blogs, different categories in the same blog, each post with both languages on it...?

It's also a free WordPress blog, so I am considering if I should buy a domain for it as for the horse related, to have more freedom on everything.

I cannot decide!!