Ciao ​@TijR. Have you ever been able to see on Strava power data when uploading just one activity?

Hi ​@Vincenzo. Yes, I see the power data of each individual activity. But when I merge them into 1 gpx file, there are no more power data.

What tool(s) are you using to merge the activities together?

When you say merge two activities, are you talking about merging the same activity recorded on 2 devices to combine the data streams, or do you mean merging 2 activities back-to-back to create a longer activity?

Hello ​@ActivityFix, I mean the second. I accidentally stopped and saved my ride before I was done. Trying to merge the two parts into 1 activity now.

Tool: I was trying 2 different tools, Gps Track Tools and Gotoes for Strava. I was merging GPX files, maybe I have to try a different type of file?

GPX doesn't store data for power. If you are a gotoes donor you can combine and export your original FIT files and preserve all the data streams. Fitfiletools may be able to do it also. 

Thanks a lot for your help all, I was able to fix the problem thanks to you