Please see Major UI/UX Update - Orchestrate Your Training - #63 by ManofSteele
TLDR: Merging is only possible if Xert has received the original .fit files from a 3rd party platform. If you’re syncing activities from Strava, you won’t be able to merge them in Xert. It’s best to configure your devices/platforms to sync completed activities directly to Xert.
The activities also need to be the same type - e.g. cannot merge weight training with a cycling activity.
Cheers!