Cisco's mobility express just runs on one of the APs and can fail over to another of the APs; it's a slick piece of software.
And yes, it isn't open source, which is a real shame since cisco's killed it (as far as I can tell) and it probably represents an enormous and sophisticated investment in effort and engineering and it'll just melt into entropy.
I loath cisco and don't recommend their kit lightly. In this one case, they seem to have accidentally made (for my use case, running 5 APs at home) a perfect product. They're cheap, extremely reliable, my wife doesn't hate them (though mostly they're in the attic or basement; only one is visible), they've got a (relatively) easy to use UI that manages all of them at once, and (Except for the switch 2) they seem to just work even though I've got vlans and lots of SSIDs and other goofy stuff).
If I had a simpler house to support, I'd just get a single WRT capable "big fast" router / AP...
I'll put in an ebay search notification for when the R650 (and R750) for $50 each and maybe it'll ding in a couple years and I'll be in a place to swap out the 3802 network I've got running now...
As far as I can tell, in this one case (with mobility express) it seems like Cisco accidentally did something overall (IMO) "good" for the world by making it possible for their kit to be reused by some part of the world where someone will get utility out of the still very nice equipment and somehow cisco won't see a dime from that "someone is using cisco equipment" event.
Mobility express isn't perfect, but considering the cost of the gear and the cost of the license, it is pretty darned good. I suspect some product manager at cisco got fired for accidentally making the world not quite as bad a place as it could be if cisco extracted license fees on all the 2802, 3802, and 4800 access points or otherwise sent them to the e-waste bin 4 years early.