You have an audio out on the Yamaha to connect the transmitter. If you buy two individual BT speakers you will need a transmitter that can pair with two devices such as this one:
http/www.avantree.com/product-bluetooth-audio-transmitter-avantree-priva-1126.html
That will give you two mono speakers if you get individual BT speakers.
To get stereo you will need to get a pair of speakers that has a BT receiver in one speaker and a wire from the amplified speaker to the passive one. This will be seen as a single stereo device so any transmitter will work.
Or consider a pair of Sonos Play speakers which don't require a connection to the Yamaha at all for internet radio. streaming services, and audio file playback on your network. A Sonos Connect would add these functions to your Yamaha. You can have up to 32 zones of Sonos, each controlled by your phone tablet or computer., so it can be expanded easily.