There are a few issues with this idea. For several reasons, it is much more practical to just get another Pi for a second Bluetooth speaker pairing. This will also result in a much reliable multiroom system, as the different Bluetooth pairings will not interfere as much with each other.
Generally, we recommend using as few players on each Pi as possible to reach the best results both in sound quality and in resilience to outside factors (e.g. bad Bluetooth signals, adapters, protocols).