Why must there be two are three agreeing as one before God will answer a prayer?

No scripture says that.

You just made that up, did you?  

The Bible does not teach so.

Citation required.

The Bible doesn't say that.  It says when two or three are gathered together in His Name, there He is in the midst.  

We can pray just as easily alone.  God has told us on many occasions He hears and answers prayer.  Look at Psalm 34 you will see it several times in there, that God heard and delivered people from fears and different things.

Also, He said, "Where two on earth agree as touching, anything that they shall ask, it shall be done by My Father which is in heaven."

You cannot take one scripture out of context and then say that's all He does.  Take ALL scriptures you can find on prayer and then see how many times God hears and answers, but in some cases ONLY WHEN WE PRAY HIS WILL (1 John 5: 14 & 15) and in another instance, God doesn't hear the prayers of sinners, only those who are worshippers and do God's will (John 9:31)

Just take all scripture together and in context.  I mean God says He will hinder men's prayers if they don't love and care for and treat their wives like they should be treated as a weaker sex so treat them gently with love etc,, (1 Peter 3:7) so all these things have to be taken into consideration, not just one scripture out of context.

Goat herders were at the top of their league at the time. I think they actually realized that with one person agreeing with themselves might be subjective. Where she might want to look up or subjective and objective. Have it that books a fairy tale