To protect against fraud in Canada do I need to go with either Equifax / TransUnion or both?

Question

As per this answer, to protect against fraud, one needs to put a credit alert on TransUnion and Equifax. Isn't it enough to put a fraud alert on just one of them? Why do we need to put it on both these organizations?

_{p.s: reason for this question is because my wife lost her purse with all of her ID cards and I set up a fraud alert on Equifax for a few $$$ / month.}