Most serial cables are straight through; that is, pin 2 on one end goes to pin 2 on the other end. Unfortunately, if you have two identical devices, such as PCs, then they'll both be trying to transmit on the same pin, and both receive lines attached to another pin... obviously, this won't work. Neither receiver is attached to a transmitter, and you could have problems because both transmitters are trying to drive the same line; doesn't work very well, HI!
A Null Modem cable just flips a couple pins so that two identical devices will get the right signals on the right pins without having both devices trying to drive a single line.
Okay, enough theory, so here's the short answer...
Get the right cables to connect one system to another. Somewhere in the line, have JUST ONE null modem cable and everything will be fine. It doesn't matter where the null modem is located, as long as it's in the line someplace. Your biggest problem might be matching connectors: 9 pin versus 25, male versus female, etc.
BTW, Radio Shack has a nice collection of little gizmos to make RS-232 a bit easier, including a null modem, double-male, double-female, etc. I would HIGHLY recommend anyone doing any serial communication stuff to buy at least one of each. The last I looked, they were all about 2" square, silver, and around $8. You might consider tossing a couple 9-to-25 pin adaptors in the shopping bag as well.
73, Bob Appelgate, K2UT