Meh, the US is different I guess. We had a second phone line for a long time and it didn't cost a dime. As for shotgunning two modems over one line, there were a few tricks way back when I was still using a 28.8k and there was one or two for 56k, but they were a pain in the ass.
Just out of curiousity how long has ISDN been in germany, in the US anyway, ISDN was the precurser to DSL, ISDN coming about a year or two before DSL appeared.