Stereo playback is NOT two speakers playing the same thing even if you have them synced perfectly.
The result would just be two MONO speakers. Stereo requires a LEFT channel and a RIGHT. channel signal playing on two separate speakers.
A MONO signal is a blend of the left and right signal, and all speakers of Alexa devices are mono; except for the Tap which has left and right channel speaker for stereo. The audio out of Dots, Echos etc are stereo outputs and when fed into a stereo amp or a stereo Bluetooth speaker the sound out is stereo. Sound out from a BT speaker when fed a stereo BT signal would result in stereo from the BI speaker.
So, if you want stereo easily, use BT to stream Echo (even audio from a smart phone) to a stereo BT speaker.