Views Filter 2 Content types but with condition on one of them
Although this post is quite old, it appears it's still an issue going up to Views 7.x-3.7. Referring to the post already mentioned here and specifically comment 36 on that page, I found my working solution.

I had two content types that I wanted to show the title from: an Event and a Blog, however I only wanted the blog posts which had a Boolean field returning as true.  Below is a screenshot of what my grouping filters look like but the trick is, on the Boolean filter, make sure you select the 'Reduce duplicates' check-box. Once I did that, I got the results I was expecting.

Hope that helps.