Uh, Flickr? Picasa Web Albums? Tumblr? In fact, any photo sharing site. Upload them and they're available to anything with a web browser. And, many of the sites have 3rd party apps that allow you to access them without a web browser - well at least on iOS, I'm not sure about Android.
But the problem is your framing the question incorrectly:
The easiest way to do this would be to back-up both libraries to the same place
You can't that anyway, to Flickr or Google+ or whatever. None of these services will back up an iPhoto Library. The best you can do is upload from the Library to a sharing service. Once framed that way a couple of things quickly beome clear.
1. iPhoto doesn't just upload to Flickr or Facebook. It has a complicated syncing relationship - if you delete a photo from iPhoto it gets deleted from the other two and vice versa, for instance - something that causes no end of confusion on here for some users. So what's actually happening is that iPhoto manages the photos stored on these services, and while I've no inside knowledge about Goole or Apple, I would note that no app - except google's own - can do that on their service. Odd that. So is it down to Apple or Google? That's Google that ceased development of their iPhoto plug-in a few years back and suggest that you use their app instead. Oddly, Apple have never done anything like that, have they?
2. As most people don't actually want iPhoto to manage the photos on these services then uploading to Google+ is exactly the same as uloading to Flickr or Facebook - via the media browser. In the case of flickr there are a lot of 3rd party uploaders as well, ones that act as plug-ins to iPhoto that give you more options that the basic uploader. Oddly there are none that I know of for G+. Now I know that there's a lot of hype about Google being "more open" about a lot of stuff, thing is, when you actually get into the nitty gritty of it, it's hard to find actual examples.