'But if a revelation be made to another sitting by, let the first keep silence. For you all can prophesy one by one, that all may learn, and all may be exhorted, and the spirits of the prophets are subject to the prophets.'
Whatever these verses mean they cannot mean blatantly that 'all can prophesy one by one' without restriction. That would be to contradict the 'two or three' on any interpretation. And it would cause the meeting to be taken over by prophecy, which would go on and on, resulting in listener fatigue, and the hearing of large numbers of prophecies that were in fact never completed because others kept stepping in. Paul can surely not mean that?
What then does he mean? The answer would seem to be that firstly he is pointing out that if a special revelation from God comes they must remember that there will be many future opportunities to prophesy, and thus prophets must be prepared to give way. For the fact is that all such prophets (over a number of gatherings) will have ample opportunity to prophesy one by one. Furthermore prophecy is under the control of the prophet. But revelation only comes more rarely and is specific. And this is said so as to justify the fact that someone who receives a revelation can interrupt a prophet.
'But if a revelation be made to another sitting by, let the first keep silence.' For there may be a time when a particular prophet receives a special revelation from God which cannot wait, and the urgency of this is such that it is seen as justifying the interruption of a prophecy. This confirms quite clearly that such a revelation had precedence (compare Acts 11:28). This was not just speaking of another 'prophecy', and it would not be something that would be happening all the time. It was speaking of a specific revelation from God, possibly an instruction on something that required doing (see Galatians 2:2). But when it did come it must be given preference.
'The first' is the first prophet as compared with 'another'. He will usually be allowed to give out his full prophecy, with an exception arising in the circumstances when another receives 'a revelation'. His prophecy might normally be followed by another prophecy, but a revelation overrides such prophecies. So if a revelation from God comes, then any prophet can be interrupted. We see from this that from time to time the early church did expect to receive such special revelations from God.