A regular serial cable won't do the trick. You'll need a null modem cable, or since you already have the cable, a null modem adapter. It just goes on one end of the cable and switches a couple of lines so that the two PCs can talk to each other.As for connecting two modems in the same room, first I think you have to put a 9-v battery across the two center wires to pull the signal up. Then when you make one PC dial, force the other one to answer.
Personally I think the null modem solution is easier and simpler.
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When 1337 hax0rs start impaling each other with swords and typing code with a hook on one hand, then they can modify the term "pirate."