It’s not the first, though i can’t think of any recent example. But if you go back to the 1980s, Murder She Wrote appeared on both Nine and Ten at the same timeslot for some time.
If you look at this thread you might find more recent examples: Different Bat-Time, Different Bat-Channel (Shows on more than one network)
EDIT: looking at that thread it appears that both 7 and 9 fought it out with The Big Bang Theory at the same time a few years back