This may need a little clarification, RichMan. It actually works perfectly for an ISO that has been split into 2 files (I haven't tried it for 2 files that didn't start off as a single file), but the syntax was muddy in the example given. Here what I got from DELL to burn a DVD of their OpenManage software:
From a command prompt, at the file location, type (without the quotes):
'copy /b file* merged_files.iso' (where both beginning ISOs start with 'file')
Copy the two parts to your c: emp\ and from that location at your DOS prompt type (without the quotes):
'file.iso.1 file.iso.2 > merged_files.iso'