The article you quote deals with American criminal law. Each state in the US defines its own criminal laws and whether a death penalty is warranted for homicide.
The key to the article you quoted is this sentence:
"Under the legal principle of accomplice liability, a defendant can be convicted of murder without being the killer. But, if the prosecutor says that a defendant pulled the trigger, it’s easier to ask a judge or a jury for a death sentence."
To start with we have no death sentence in Canada.
Next our Criminal Code does not differentiate between the man who holds the gun and fires it and the man who actively aids the other shoot the gun. They are both equally as guilty do the phenomena you discuss does not happen in Canada.
If you can find Canadian cases where this happened I would love to know them because I am not aware of any for the reasons I stated
What is more probable is that when two persons are engaged in a murder, one turns witness for the state and gets their sentence lowered to criminal negligence, manslaughter, an accomplice before and/or after the fact but its their sentence in terms of prison time that might be reduced as part of the plea bargain to testify not their actual being guilty.
This doesn't happen because our criminal sentencing structure does not provide any reason to do the above in favour of a defendant.