Of course we are dealing with some unchartered 4k territory here, so there's a learning curve, and that alone makes it exciting. Regarding your comment, well I'm doing what you think is almost certainly not possible!
That is, both Seiki displays are HDMI only for 4k. And on this Sapphire card, thru the native HDMI port connection I can display 3840x2160 @30Hz. Alternatively, on the display port output, with the use of an Accel UltraAV Displayport 1.1 to HDMI Passive Adapter I am also displaying 3840x2160 @30Hz. The card itself offers DP 1.2
The specs for the Accell adapter state "Compliant to HDMI 1.4b, throughput up to 300MHz. Ultra HD 4k x 2k, stereoscopic 3D and Audio. This Type 2 adapter supports 1080p, 3D, and 4k video". I bought that adapter specifically for those characteristics, as many adapters do not state 4k support.
As mentioned when I connect both displays, each port, HDMI or DP, will display 4k on one monitor - but the resultant output on the other port becomes 1080p. I can switch back and forth whether HDMI or DP is in 4k, but each time the other monitor switches to only 1080p.
I can of course just (with $200) get another matching card, but before that, I'd like AMD to tell me officially that one HD7870 card alone can not support 4k on two monitors simultaneously.
Of course we are all just shooting in the dark, which is interesting, but I had thought this forum was where you got official AMD support but apparently AMD either does not monitor posts or ???, as no definitive answer has been forthcoming. I tried calling support but after 45min on hold with a repeating message to wait, I gave up. Frustrating.