From the product product specifications: These features allow you to setup multiple SRS-XB21 (or compatible speakers) together in a chain or specify their position individually. You can add up to 100 speakers it says but I have no way of testing that. It is nice to be able to add 2 of them and specify one for left channel and one for right. Here is a quick list of compatible speakers - SRS-XB21, SRS-XB31, SRS-XB41, SRS-XB20, SRS-XB30, SRS-XB40, GTK-XB60, GTK-XB90.
Are your hot tub speakers on this list above? If not, the answer is "no". But the Bluetooth radios on these units have a good range, and they are portable, so you could simply take the Sony to the hot tub.