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...