A single 970 is enough to run solid framerates without DSR at very high settings. That is what I run, and no issues at all. No jitter or other lag issues.
But the moment you turn on DSR even at low additional resolutions then it is not enough. I'm sure you can then start turn down quality, but that defeats the purpose.
I tried DSR in Elite (ignoring the lag for a while) and did NOT find it worth the extra card. So I run a single setup with default resolution.