Hey I googled into this a bit. Cant say I ever played this special version of the game though. So I found this web page http://www.fortunecity.com/westwood/dior/3/ma … /marcus-bi.html
where they say that there is some bugs in the original version multiplayer and you have to patch it. sadly the link for the patches on that page didn't seem to work. so I think it would be a good idea to google into it some more and try to find the patch and have a new try with the multiplayer. the modem should work with dosbox, and the second is basically just a play by email system so it should be even simpler when the computers share a directory.

So cant help much but that's something to dig deeper into. Hope this is of some kind of help for you. good luck with it.

Heh well I couldn't stop digging deeper myself and found at least this site with some patch for that game. it's in german though http://www.bluebyte.net/ger/downloads/default.asp?id=''
Cant say if it helps though. just guessing 😀

Hehe... /me happens to be a german guy 😉
I thank you for your help... but with patch the game won't run anymore. Strange. I still have to figure out how that email mode works...
Damn ebay... got those CDs cheap but without any manuals 😒

Well hard to say. Seems that it is a rare edition only released in europe.. Yet still there is a english and german edition. So maybe your patch is for the wrong one. Dunno, couldn't find much more than that on google though.
Half of the links were out of use and stuff.