Both the Wave SoundTouch IV and the Sony BTX300 speaker have Bluetooth capabilities, but they both only support Bluetooth Input to the system, not Bluetooth output. You would be unable to pair these two speakers together because of this.
Having said that you are able to pair multiple SoundTouch speakers together. The SoundTouch product line can use your home network to join the two speakers together using our Grouping or Play Everywhere feature.
For example if you have a non-SoundTouch product with an AUX connection such as the Sony BTX300, you could connect a SoundTouch Wireless Link to the AUX port to enable that speaker to have SoundTouch functionality and have them both play together.\
Let us know if that helps answer your question! Thanks!