I assume you have the two cards placed very close together.
The top card is getting choked by the lack of clearance between the fans and the bottom card. I would recommend you to remove the back plate of the bottom card to increase the clearance between the card (you won't sacrifice the aesthetics because the back of the bottom card is sandwiched in the middle, which you can't see it anyway). With the few precious extra few millimetres of space, the fans of the top card should be able to draw in some cool air, hopefully and ideally you should see some temperature drop.
To further improve the temps, add a side fan that blows air onto the sli setup from the side should be ideal solution, as it is the side with the largest exposed heatsink will be and the place for the gpu fans to suck air in.
Think about it, remove the bottom card's back plate is a very small sacrifice for aesthetics but a large improvement for your gpu temps and can prolong your components.