While Replay can create a continuous, automatic copy from the master store to the destination store(s) on a real-time basis, it is ultimately a one-way copy. This means that for all items, you will not be able to have a two-way sync between the two shops.
Replay is simply for copying items that get sent from the source shop (the "master" store) to additional shops (the "destination" store(s)). It is not a continuous loop where data can be sent back to the source shop, and thus be kept 'synced'.
Therefore, at this time, we do not recommend attempting to setup a two-way sync.
This is something we are working towards for all copyable items in the future.