The process that SIGNALS uses does work, however, unless there has been a recent mod to the software, you are limited to only 1 audio track being merged. If you select 2 audio tracks for inclusion in the final file, you wind up with 1 merged track and then 2 more tracks listed, the second track from file one and the second track from file 2 for a total of 3.
If you only want one audio track in the final (merged) file, this is a good way to do it.
Also, if my memory still works, I believe you cannot use the MKV passthru  profile when doing this as DVDFAB must re-encode the files.
If you want more than one, there is a solution, but I am not sure I am allowed to post that procedure as it involves using a free 3rd party tool as well as the DVDFAB software.