If TWO of you agree, it will be done for them by my Father in heaven

[tonychanyt]

By itself, is this a universally true conditional proposition?

No, experiences tell us no. Let's see the context, Mat 18:

15“If your brother sins against you, go and tell him his fault, between you and him alone.

That's Step #1.

If he listens to you, you have gained your brother.

Step #2:

16But if he does not listen, take one or two others along with you, that every charge may be established by the evidence of two or three witnesses.

Step #3:

17If he refuses to listen to them, tell it to the church. And if he refuses to listen even to the church, let him be to you as a Gentile and a tax collector. 18Truly, I say to you, whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven.

Jesus was talking about the importance of two or three witnesses. Similarly:

19 Again I say to you, if two of you agree on earth about anything they ask, it will be done for them by my Father in heaven. 20For where two or three are gathered in my name, there am I among them.”

Jesus applied the two-or-three-witnesses rule to praying. There is a second condition: "in my name". If the two or three of you agree and if they pray according to Jesus' name, i.e., according to Jesus, then it will be done for them by the Father.