I have never thought that because I can't see any scripture to support it. Whatever thing I ask my friend to pray about is what I am also praying about. If that thing is within the will of God, then I know it will be granted, no matter how many people are praying for me. If it is not, then it will not.
FK: "... when you pray to Mary: "Mary save us!", what exactly do you expect Mary to do for you?"
I expect Mary to pray for me to Christ, so that He is merciful to me should I falter.
I mean, I'll take your word for it and all. But it sure doesn't "sound" like that's what the prayer is about. :)
It appears you have no good reason to ask him to pray then, as the outcome is predetermined. But St. Paul asked for others's prayers, just to give one example.