I think for SEO having two languages doesn't matter but I would make sure a single page has everything in one. That looks more professional to visitors too.
Are you going to use the categories across the languages? If yes, then what language are you going to use for the archives if there are articles from both languages?
I don't know if it's true but I've read that the domain name may be taken into account by search engines. So with 1 blog you have 1 domain name for both parts, where 2 separate names might be better.
My site is in Dutch, but 1 category (Programming) contains articles in English. I had to do some programming to get everything in that category in English. For example the correct English month names in the publication date.
So mixing is possible, but may need some extra care to set up. I don't know if there are themes or plugins to help, but I didn't really search much for them.
Hope this is useful,