That depends on the seconds that have past and will pass after the last monsters turn before the death sentence is about to go off. That sounds a little bit like nothing, but here’s how it basically works:
The death sentence timer will go off after 320 seconds after its use, right?
You use it - 320 timer is on.
You’re opponent uses it (lets say 10 TU seconds later) 320 timer is on, while yours is now on 310 seconds left.
Seconds will pass, and lets say this is the following possible situation:
You’re timer has only 20 seconds left to go off. This means your opponents monster has 30 seconds left before it goes off. But if the next monsters turn is 50 seconds TU away, that means it’ll trigger both because both timers have run out.
It’s all about narrow seconds in this case. Preventable, but you sure need to calculate it all the way through and there’s quite some luck involved.