Faith and Asking

²⁴ Therefore I tell you, whatever you ask for in prayer, believe that you have received it, and it will be yours. God's language of faith is present and past tense. When you pray, believe God for it and thank Him for it as if you've already 


'The Hebrew writers often express the certainty of a thing taking place by putting it in the past tense, though the actual fulfilment may not take place for ages. This is easily understood and appreciated when the language is used by God, as when He says, in Gen. xv. 18, "Unto thy seed I have given this land;" and in xvii. 4, "I, lo, My covenant is with thee, and thou hast become a father of a multitude of nations."
http://www.ccel.org/bible/ylt/ylt.htm'


Matthew 18:19
"Again, I tell you that if two of you on earth agree about anything you ask for, it will be done for you by my Father in heaven.

2 is the smallest plurality and the church is there even when there is two. 
The key word here is agree. If somebody believes that they would be cured by surgery, then pray with them for good surgery, for a painless recovery so that they can agree with you. Don't impose your ideas on them.