As long as the base card is the same (GTX 970 4GB) I think you will be OK to SLI them.
Could you research it for me as im not good at it ?
i've done enough research on graphics cards the last twenty years to confirm as long as both cards have the same GPU, frame buffer and clocks SLI will work fine. If clocks are different you can use MSI afterburner to sync them up but you'll have to likely sync to the slower card's speeds for stability.