This is what I did. FTP your Original game, either disc, to your HDD. Both discs are indeed identical save for the .TXT file name.
Now, delete the "default.xbe" file. Yes, delete it. Now, rename "Doa1.xbe" or "DOA2.xbe" depending on what game you want it to be to "default.xbe". This will cause the disc to load the game you select directly.
One other note. When you select Doa1 or Doa2, you will only need the corisponding folder - meaning - if you rename "Doa1.XBE" to "default.xbe" you will not need the "DOA2" folder that you FTP over, thus making the game <300mb. Same goes for if you rename "DOA2.xbe" to "default.xbe" you can eliminate the "DOA1" folder.
I actually made it a multigame disc putting both games on the same disc. I made 2 folders, one for each game and copied the FTP'd files to make 2 identical folders. I just deleted the "DOA2" folder and renamed "Doa1.xbe" to "default.xbe" for the first game, then for the second game I just renamed "DOA2.xbe" to "default.xbe" and there was no need to delete the "DOA1" folder as the multigame .iso was 4.28gb.
I have put about 2 hours into DOA2 and about 15 mins into DOA 1 with no problems. <