I have found that when using two single zone triggers in the same input (i.e. splitting a single 1/4" input into two single inputs) that there is a built in "x-talk" factor of sorts that can't be changed. This makes it difficult to fire off those two triggers at the same time. I was able to over come this by using two separate physical inputs on the I/O. I discovered this when I was trying to trigger both zones from a single input using only one piezo element. Both sounds would not trigger at the same time. Once I split the signal of the one piezo element over two separate inputs, I was able to trigger both sounds simultaneously.
I know to most people this will never be an issue, but with some of the DIY stuff I do, I kind of stumbled on it.