When using a keyboard shortcut to trigger the "Frontmost Windows Side by Side" action, both the frontmost windows end up on the same side on top of one another.
MacBook Pro, 14-inch, 2021, M1 Pro, 16GB
Sequoia 15.0
BTT Version: 4.725 (2024092705)
Are you using the native snapping integration?
If so, BTT triggers the menu function Window => Move & Resize => Left & Right. Does that work if you trigger it manually for these windows?
I was using native snapping integration, and manually triggering the Move & Resize via holding the green dot on the top left of the window works. The Frontmost Windows Side-by-side action works only when using the classic BTT resizing setting.