The ATI 6000 series supposed supports DisplayPort spec 1.2 which supports Daisy-Chaining. However, I'm not sure how it would connect in your case as each of your monitors only contains a single DP connector.
You're probably best off using one DP and one DVI connection.
Original August 13, 2013
I went back and forth with Dell Technical support to try to solve this problem. They were attentive but ultimately unhelpful.
Get Better Cables
I was using the male-mDP to male-DP DisplayPort Cables that shipped with the U2913WM monitors to connect them. They look like they are good quality. On a hunch I replaced them with two Startech DISPLPORT3L Cables. They are heavier, shorter and use the full-size DisplayPort connectors at both ends of the cable. Since changing to the Startech cables (about a week ago) my monitors have not entered power save mode.
Update May 16, 2014
While the cables helped for a few weeks, the problem eventually came back and became progressively more frequent until it was, once again, happening multiple times per day. Once it happened daily again, I methodically made a single configuration change each time I rebooted. The changes included, among other things, disabling and enabling nVidia Optimus, testing multiple BIOS and driver versions, introducing an MST hub, changing the cabling topology, etc. Most changes did not impact the problem. Some seemed to subtley change its characteristics.
Reduce Refresh Rate
Finally I found one thing that made an unambiguous improvement: Reducing the refresh rate of the monitors. The only refresh rate that the Dell U2913WM monitors report to Windows is 60Hz. Using the nVidia control panel, I created a "Custom Resolution" of 2560x1080 at 30Hz (32-bit), progressive. I was able to confirm the reduced refresh rate using the monitors' on-screen display.
The monitors have not entered power save mode since the reduction in refresh rate. It's too early to tell whether this completely fixed the problem, but I am now on day two without a monitor shutdown which is already a huge improvement.
Fixed related issue: At 60Hz refresh rate, Windows would sometimes not allow me to select the full 2560x1080 native resolution of both Dell U2913WM monitors. Specifically, 2560x1080 would be missing from the list of available resolutions in "control Panel"=>"Change the appearance of your displays". Reducing the refresh rate also seemd to fix this problem.
Update June 25, 2014
It has been five weeks of daily use since changing the refresh rate of the monitors from 60 to 30 Hz. In that time, the monitors have never unexpectedly entered power save mode. I consider this to be solved.
Best Answer
OK, an embarassingly simple solution - only one of the two DisplayPort cables was connected, so only 4k resolution was available. The UP2715K on-screen display shows two DisplayPort icons side by side, but that doesn't necessarily mean that both ports are connected, only that it will try to use both if possible.
So you can successfully drive two UP2715Ks from a single NVS 510 card.
Since posting this question I have installed a second NVS 510 card in the same machine and am driving a third UP2715K from that second card. I expect that driving four of these monitors would be possible from the same two cards.