Pretty much as the tile says, what will happen if two users are reviewing the same item?
Is it possible at all, or when one opens a review item, is the reviewed item not available to anyone else while he/she is reviewing it?
There shouldn't be any parallel reviews of the same post. From here:
An optimization to the Late Answers, First Posts, and Low Quality review queues has been deployed that will keep a single review from being shown to multiple people at the same time. This should help the situation where multiple people would perform reviews which resulted in multiple up-votes. I will continue to monitor these queues closely.