You divide the computations equally among all the gpus.
The speed and memory of each gpu in the set is equivalent to the slowest one.
This happens because the batch output is synchronized in the end. Therefore, even if the 1080 ti finishes earlier, it will wait for the 1080.
On the other hand, as the batch is equally distributed among gpus, the 1080  will get out of memory before the 1080 ti.