Multiple errors. Suggest that you approach it this somewhat convoluted manner...
Place one source file on the timeline. Export it as an mp4. Clear the timeline. Place the other source file on the timeline. Export it as an mp4.
Clear the bin of the MTS files and load it with the converted mp4s.
In the bin, click the first file. Under the preview window, set IN/OUT points with the red and blue START and END brackets.*
If the same clip is to be used for other segments, right-click on it and COPY. Then right-click an empty area of the bin and click PASTE (clip).
Set new IN/OUT points.
When all segments have been made into individual clips, drop them onto the timeline. Use ADD (under the preview window), which adds them to the end of existing clips, right-click on each and PLACE ON SEQUENCE AT END or drag them individually.
When done with the first file's clips, click the second file. Set IN/OUT points and follow the same procedure.
Apply the speed up to the desired clip on the timeline.
-0-0-
If the speed change results in flash or unwanted frames, follow this procedure...
Speed up the clip, then right-click it and COPY
Click the + sign at the right side of the SEQUENCE ONE tab above the timeline, to create a new, blank sequence 2.
Right-click in the sequence area and PASTE (insert).
Export sequence 2 as an mp4.
Click the SEQUENCE ONE tab.
ADD the newly created sped-up file to the bin.
Drag it to the timeline, directly over the original. It should replace the original.
Your result?
* After clicking the AUDIO tab on the toolbar there's a "scrubbing" item. Confirm that it's turned off.