Pc build sli gtx 1080ti

Well you might if you put them in a shoebox with no fans. Can't really say without a full part list.

But you are looking at a lot of heat being generated. Something has to take it out of the chassis and something has to bring in cooler air.
 
You will need some serious airflow in that case. I got 2 FE cards in right now. Top ones sits at 80c 100% fan while the bottom card gets around 70c with 80% fan under 24/7 load. Its a little better with a none blower cards, but than you really need to find a way to dump the air. If you are just playing games, I don't see a problem. However the top card is going to be upwards of 10c over the bottom card. Heat raises.