No way a BT speaker can play to another BT speaker. You need a transmitter and a receiver, not two speakers.
Nonetheless, There are lots of posts about connecting a BT transmission and Echo speaker together in this forum. (Basically, you need an app on phone that plays music (from any source) and then you use the phone or other device ( a PC) to send BT music to the Bose, or Echo.
Please search the site messages for the word Bluetooth, and then enjoy reading the posts that appear.
By the way, if the Echo had a headphone out jack, you could connect a small BT transmitter to the
Echo headphone out jack and send the output to the Bose speaker. (This is exactly how I send my iPod music to anything via BT; like to the Echo, BT speaker, etc.) There must be a reason Amazon didn't put an output jack on the Echo; maybe they don't want us to be able to bypass the Echo's marginal quality speakers. Some people have tried to hack the Echo to have a headphone jack, but the problem is the Echo does mot have stereo speakers, instead it has hi and low frequency speakers; not at all same as stereo.