Media diagnostics · Primary-source explanation; examples are illustrative. Instructions are based on publisher guidance; a UK IPTV account was not tested on this platform.
Describe the desired result first
Two local clips can be combined in different ways. A second commentary track should sit alongside a video, while a second part of a programme should follow the first part in time. The mkvmerge manual distinguishes adding input streams from appending a file, with explicit append syntax and track mapping. Selecting the wrong operation can produce an unexpected track list even when the output file is valid and opens normally.
Count tracks and time separately
In an original conceptual example, each of two compatible parts contains one video track and two audio tracks. Appending corresponding tracks aims for one longer video timeline with two corresponding longer audio timelines, rather than six unrelated parallel tracks. This is an explanation of the intended mapping, not a guarantee that arbitrary files are compatible. Inspect the inputs, their track identities and properties before deciding how they should connect.
Verify the join itself
Keep separate source files and a new output. Check the transition around the joining point, the total duration and each intended language or commentary selection. The manual describes timestamp behaviour and mappings which can matter when parts have different track endings. Do not assume that a successful process exit establishes a perfect audiovisual join. This operation does not translate tracks, repair missing programme sections or provide permission to combine recordings; it controls how permitted input streams are arranged in a container.