It is possible when asking another person to pray for yourself, to be committing serious spiritual sin.
In order to discern if one is acting properly, ask oneself, “What and who is the object of the prayer?”
We are to have faith alone in Christ alone. We have salvation through our faith in Him. We are to pray continually, always, again through faith in Him and no other. We are each our very own priests to Him through faith in Him who is the High Priest. Our prayers are sent through the unspeakable groanings of God the Holy Spirit who makes them understood, knowable and presentable to the perfect holiness of God the Father. Through faith in Christ, we have direct access to God.
There may be temptation to pray as a spoiled child, recognizing God is all powerful, and loves us very much, and in so doing thinking we might be able to get our wants delivered to us by Him, if we simply pray in an egotistical fashion to needle Him into performing our will. Some might go so far as to encourage others to also pray for them so as to cause a distraction to God so they might get their way.
This type of prayer places faith in the person and his plan before God and His Plan. Accordingly, that type of prayer promotes spiritual sin, instead of faith through Him in all things.
I have not investigated where it might be possible, if possible, to request others to pray for ourselves while remaining in faith through Christ.
It is obvious we may act as an assembly, a congregation, a Church, to pray to Him by His will, focusing through faith in Him. As such, we are so directed in the Lord’s Prayer.
If one prays to a saint, it would only be legitimate if it were to be prayed through faith in Him, but if only between the person praying and the saint being prayed to, it no longer is holy but becomes sin, even additioanlly a sin of seeking familiar spirits.
Over on HarleyD's "Calvin on relics and such" pages I was suggesting that we are all, without grace, idolatrous, and that we can even have an idolatrous relationship with God Himself.
And to the extent that "simul iustus et peccator" accurately describes our fix between the cross and the kingdom, idolatry will ever be at least nipping at our heels.
One of my reasons for starting prayers with making the sign of the cross and murmuring "In the name of the Father and the Sone and the Holy Spirit," is that I know I can never do anything right without God's help. So it's an appeal that God will bestow his grace on my prayers and supply themn with his sanctity, since nothing I can do on my own could possibly be holy.
And similarly, I end my prayers withthe same little act as an appeal that God will continue to supply the remedy to the gaping holes and defects in my prayers and intentions.
It's important to add that I'm confident that God in His mercy answers those prayers and resolves those intentions. "His mercy endures forever."