combine multiple feeds with fetch_feed and display blog titles for each item?

I don’t see anything to suggest that fetch_feed is supposed to accept an array of values. That is not mentioned in the Codex, nor the source, @param string $url URL to retrieve feed.

That said, fetch_feed passes things off to SimplePie’s set_feed_url($url); which does accept an array. And that Simplepie docs page is, I think, where your answer is. The RSS feed name is $item->get_feed()->get_title() not $rss->get_title()

I am still a bit cautious since you are passing an array through a WordPress function that doesn’t appear to be meant for it.