It always falls even further back - when someone answers you with that passage, ask THEM:
"Well how do we KNOW when two or three people are really gathered in Jesus's name? Do we just take their word for it? After all, EVERY Christian that I know says they are gathered "in Jesus's name", and yet we have thousands of different denominations in the US alone, who cannot agree on a uniform interpretation of the Bible. Now, how can Jesus allow MISINTERPRETATION to occur, IF they are gathered "in his name" and he is really "with them"? Do they REALLY believe Jesus would be "with them", and then NOT correct them if they erred in their interpretation?
You see, questions of that sort will ALWAYS presuppose a "given". The key is to learn how to spot WHAT they take for granted as a "given" and then turn the question back around on them and ask THEM to explain their "given".
Again, in your case, their question is PREMISED on their claim that "two or three people can meet in Jesus's name, and then whatever they decide, it must be correct". Well, that conclusion presupposes that ANY two or three people can meet, and that ANY two or three people have authority of some kind to meet IN JESUS's NAME. So ask them: HOW does one know when someone is meeting in Jesus's name?
If they claim that ANY two or three people can meet and just SAY that it is in Jesus's name - and that is sufficient - then that means Jesus has either lied (which we know he cannot do) or Jesus did not mean what he said literally, that is, "ANY" two or three people who get together and say, "we are meeting in Jesus's name", must make it so.
Well of course that does not make it so . . . THAT is why we have the proliferation of protestants that we have - they ALL claim they are meeting in "Jesus's name", and yet they continue, 500+ years later, to invent new interpretations, rules, doctrines, etc.
I am certain - Jesus did NOT mean that, when he said that verse . . .