shafat
I recently purchased two gtx 980 Sc acx2 and decided to put those bad boys under water for further overclocking. Before I get to the point of this thread i must confess that I am overwhelmed by how cool and efficient these card run with liquid cooling. My idle temps are ~29-31c and absolutely nothing over 40-41c under load (Fire strike, Crysis 3 DSR 4k, Metro LL). 
 
Now to the main point, while I was doing my benchmarking and stress testing, I occasionally ran into minor stuttering and display driver stopped responding error, something I had seen before with my 580s if their oc wasnt stable. Now keep in mind that I have not yet oced my cards yet. After few nights of troubleshooting, I noticed that in Afterburner, my gpu 1 and 2 load voltage were completely different. Gpu 1 @ 1.182v and gpu 2 @ 1.212v (<---- normal) during full load on fire-strike. Idle voltage were same (.866v on both). Now this seemed odd to me so i unhooked sli and ran both cards individually. To my surpirse, both cards ran at their full 1.212v when ran individually. As soon as i slapped the sli bridge on and enabled SLI, all went bonkers. I believe these voltage difference were causing the crashes during benchmark.
 
After some google searching, I ran into a thread in geforce forums where peeps were discussing similar issues and one member came with a temp solution to this issue. Basically, he suggested that the card with the higher voltage during load should be ran ~20-35mhz higher than the lower voltage one. In my case, since my gpu 1 had lower voltage, I added +25 mhz to my second gpu and boom, my load voltage on both gpus are now 1.212v. So far I was only able to do testing on Crysis 3 (haven't run firestrike yet). 
 
Here's the link https://forums.geforce.com/default/topic/777448/geforce-900-series/-major-issue-sli-gtx-970-cards-one-card-runs-way-lower-voltage-than-the-other-driver-bug-/5/
 
I am not trying to cause a panic or stir up a hornets nest here, just posting a observation I had. I hope a Evga or Nvidia rep can further clear up my concern.

shafat
I understand the concept of ASIC. My ASIC for both cards are 74.8 and 76.2 respectively. However, I do not believe this slight difference in ASIC number result in over 20mv in voltage difference during load. Also, the "solution" to this problem as stated by geforce forum member is kinda counter intuitive, where increasing the mhz of the higher voltage card by 25mhz makes the lower voltage card bump its voltage up to 1.212v which it should be in the first place. Also, how come when the cards are ran individually, both of then use 1.212v during load and not do the same in SLI?
 
Blows my mind.