Bluetooth 5, Multiple Speakers, and iPad mini 5 on iPadOS 14.5.1
Your issue is not with bluetooth but with iOS's audio out selection.
Your device is connected to both devices as both are present as options in the airplay screen as seen in your image. You can confirm this by going into bluetooth in settings and seeing that both devices are connected.
The issue you have is that iOS can only output to one device at once, with the exception of the airplay compatible devices (e.g. Apple TV, HomePod, 3rd-party airplay 2 compatible devices, etc.) or headphones/earbuds with and apple chip, either W1 or H1 (AirPods &amp; beats).
So the only way at the moment to get multiple bluetooth speakers playing audio from your iPad is to buy one that are airplay 2 compatible.