Both of you need to download and use
MultiSH4. With it, you can rename the Save folder for each of your copies of the install. Unzip MultiSH4, and put a copy of it in each of your install folders and run it from there.
What I do Niume, if I'm not using Rockin Robbin's
Corruption Management System, is I take my base SH4 folder, such as "C:\Games \Ubisoft \Silent Hunter...", and I select and copy the "Silent Hunter..." portion (along with all of its sub-folders and files), and paste it to another place on the hard drive. Once copied over, I rename that new "root" folder to something suitable for the mod, such as "TMO" for a TMO modded SH4 game. Then I copy that "TMO" folder back into the "Ubisoft" folder. There are now basically two identical Silent Hunter 4 games in my Ubisoft folder, but only the original has a short-cut to start it with. Run MultiSH4 in the "TMO" folder, change the Save folder from "SH4" to "TMO", save and exit from MultiSH4. You now have two separate games that (in theory) won't interfere with each others' Saved games. Now activate your mod, and then create a short-cut for the "new" install.
If you have a "clean" SH4 install, you could use 7Zip or some other archive program, and "zip-up" the "Silent Hunter..." folder into an archive, and that makes moving things a bit easier. You might re-name the folder before you zip it...
