When it comes to safety, it's better to err on the side of caution.
Turn the question on its head - rather than asking why shouldn't you, ask why should you? Which is safer? If you're putting a shoot on expecting to have lots of stoppages, spend a bit more time preparing and testing your targets. Make sure bolts are tight, strings are double tied etc.
Here's a couple of scenarios for you :
First off, someone on the course hears a whistle and isn't sure it means they need to stop shooting because they're confused about which part of the course they're on.
Secondly, course A stops correctly, and course B carries on (correctly again).
Someone on course A hears shooting going on and assumes the ceasefire is over but they didn't hear two whistles.
I can think of other scenarios that could be potentially dangerous too and the only reason I can think of to take such a risk is to save a couple of minutes delay. Is it really worth the risk?
My opinion is no, taking risks with people's safety is never worth it.