Power meter disconnected from head unit mid ride today so I stopped the session and restarted once pairing had sorted.
Does anyone know how I can merge the two activities?
Thanks 
Use FIT File Tools to create one file, import, and delete the originals.
I have also used the site to strip power on occasion.
You may want to do this if your power meter battery conks out part way through a long ride.
You can then enable HRDM to estimate XSS.
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There is a tool available that works with Strava called FIT FILE TOOLS
Steps
1- web search “fit file tools”
Looks like this
connect with Strava (optional)
2- download fit files that you want to combine
3- from fit file tools page launch combiner and follow instructions
4- once new combined file is created upload to desired app (I.e Strava, xert, etc…)
5- review new uploaded file to ensure it is accurate, if reject, delete file in apps and go back to step 3
6- delete original files
7- (revised) review results, notify support if something looks off
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Thanks both 
Note this step is likely unnecessary. Progression recalcs should only be performed when there are significant errors in your progression data. You can always message support for assistance with these sorts of issues and someone (myself included) would be happy to assist! 
I’ve run into this same issue when my head unit dies mid-ride and creates two separate files. It’s a bit of a headache that there isn’t a direct ‘merge’ button in the dashboard yet. Most people end up using specialized fit file tools, but honestly, if you just need to stick them together quickly, I’ve used Join Files Online before. It’s super straightforward and works well for combining files without needing to install anything heavy. Just merge them there and then re-upload the single file to Xert!
It is possible to merge two or more activities in Xert now.
Yeah, that’s a frustrating situation.