I'm sure it was done before many many times, maybe not with those specific speakers. I've done it with a piano to split audio to two powered mono speakers.
Nice job getting it done and great illustration!
I just tried this split with 3.5mm Aux cables for a party. I was just trying to push more sound for a house party and happen to own two Soundlinks. I didn't need true stereo sound division so it made sense. (side note: I own a full sized entertainment system (two toweres, center speaker, 10" sub) but hauling that to someone's house would have been a pain))
Result: It Worked!... but it wasn't the loudest. In my case, I think splitting the singe source audio caused a power drop over the split. I did sound comparisons between my now $500 dual Soundlink rig and my $50-$75 Altech computer speakers, and the computer speakers won hands down. I know it isn't a fair comparison since the two speakers are designed for vastly different purposes, but when you are just trying to drive a lot of sound out of something, apparently you are better off going for dedicated computer systems. By the time I bought an audio splitter and extra cable to set up the rig. I could have just bought a mid range computer speaker rig for the party.
Either way, I think is a great solution, just not the best if you have other options.