The race has already began! League TWO is running away from League ONE, L2's top
3 teams have more points already than L1's top
6 teams!

XS, [H], and EVGA are really tearing it up thus far!
GTX1080Ti with 3 tasks is about 22m45s avg, so a bit more ppd there (vs 7m50s for single task), also no idle time waiting for the next task to start, Windows10.
GTX1080 with 2 tasks is 21m30s, 3 tasks is 32m00s, Windows10
GTX1080 with
2 tasks is 23m00s, 3 tasks is 34m30s, Linux.....hmmmmm, much worse than the above system, but perhaps the 700MHz less CPU clock is at play.
Old R9-280X with 2 tasks is 19m40s, 3 tasks is 29m00s, Windows10. Note:
Much lower CPU required, vs one thread per task for Nvidia. One thread was needed to run 4 GPU tasks (occasional spikes though).
Long story made short: There is a definite advantage, although small, to running dual tasks per GPU, not real sure about triple tasks though.