The Yoyo (M)'s will indeed play in stereo via Bluetooth.
Once you have connected your compatible Bluetooth device to the Primary (right) speaker, the signal will be divided between both Yoyos to produce a true stereo sound.
If you decide to connect to the Yoyos via an auxiliary cable, the speakers will then play as dual-mono sources. This means that both speakers will play exactly the same sound. This is because the Primary speaker (right) will duplicate the signal of your connected device (phone, laptop etc.) and send this to the Secondary speaker (left).
Please note: The Bluetooth and Auxiliary connections must be made to the Right speaker as the Left speaker can only receive information.