I really haven't used ScidvPc much at all, so I don't have much of a working knowledge of it. But I think I know enough to give a basic answer. The program can't merge two pgn files. Scid CAN change a single-game pgn file, but if a pgn file consists of multiple games, it can only treat it as read-only.
So you'll have to do this in one of two ways - (1) Either merge the pgn files manually using a a text editor meant for large files (or a pgn utility program that can merge pgn files), or (2) convert the pgn files to Scid format first (see the Help file), then merge the Scid files. Once you have one big Scid file, you can always export the Scid file back to a pgn file if you want.
I've been using this program for a while now but my understanding of it is very limited and I find it REALLY confusing.
I downloaded all my live games in 9 PGN files of 50 each (I can't get the site to download all at once, which would solve the issue) and I want to combine them all into one database that I can search with scid.
I thought I could just use the export pgn option to add them all together, but it doesn't work. When I try to add two PGNs each containing 50 files I end up with one PGN containing two files - one from each.
I eventually figured out that I could drag and drop each PGN file onto the database switcher, from windows explorer, but when I try to add them I'm told they are read only and can't be changed.
Could some kind person give me step by step instructions on how to build a 400+ database from 9 read only PGN files? I think I'm close, but so far have no bananas!