I dont know how much RAM you have in your Amiga, but if you have - unlike me - more then one meg then you should copy the smaller file to "RAM:", then join the bigger file with the smaller one which is already in RAM and the output should go to "RAM:" also.
what you'll need is to use a dos-command (takes less memory then a GUI-progg), possibly the "join" command but I havent tested it and dont know if it works with binary files, if not, I'm sure there are dos-commands to download from the net (aminet perhaps ?).
Anyway, if you have MORE then 1 meg RAM then your a lucky guy, if I had more ram then 1 meg I would be playing my old games right now :-(