iirc, the sli is only doablebetween two identical gpus
since one is a 1080 and the other is 1080ti it is possible it will not work, theoretically they share the same gpu and are very near but is not guaranteed to work
also remember that this days some games will not allow you to use sli, some games will work a bit weird, only some will work very well on sli cards, most times one gpu is not used, only one of the two works