Fortunately one 290 should run Heroes of the Storm fine.
I don't have AMD, does the new Crimson interface make it easier to turn CR on and off? For some things I think it now supports game by game settings which may be very useful.
A lot of people swear off SLI/CF after they have tried it as it is often not supported or supported late after the release or just more bug prone etc.
The coming years may see a turn around and make multi-GPU a realistic and useful thing to have.
1 - DX12 means the game developer can develop multi GPU support independently from AMD/Nvidia. In theory this should increase the games which come with multi GPU support from day 1. Though unless it is very easy to do I would still only expect it on AAA games or games which are graphically demanding (often the same thing)
2 - DX12 should be able to make more efficient use of the GPUs and by the way it works make them less buggy/prone to problems as well as being better at scaling up with more GPUs.
3 - VR. At the moment* VR produces the image twice (one for each eye). I think the games also need to run at 90fps for a bunch of reasons. So you need some serious horsepower, a need which will only grow as the resolution on the headsets grows. Elite Dangerous, which recommends a GTX 770 on Steam requires a GTX 980 minimum for HTC headset (As well as 16gig of RAM). (http://www.tomshardware.com/news/elite-dangerous-vr-specs-revealed,30696.html)
So over the next couple of years I think multi GPU systems should become more viable/less of a headache and have more things to stretch their legs on.
For Ashes, I would just be patient, it shouldn't be too long hopefully. 2x290x's under DX12 should be very nice indeed.
* I say at the moment as I think I saw some write up of a tech in the works which somehow gets away with only producing the image once. Not sure though.