You will need a null-modem cable to connect between the two serial ports of the machines. The null-modem cable is essentially a serial crossover line (signal xmit is on pin 2, signal receive on pin 3, and ground on pin 5 for each machine so it needs to be crossed over to have xmit on one connect to rcv on the other). Then set up the control panel/modem utilities menu for "dial up networking serial cable between 2 PCs." After this is done you should be able to set the RAS settings on each PC to accept the input of the other. Remember that the serial speeds and other settings (parity, etc.) also need to be the same, but you should be able to work up to 115200bps if both PCs will support that speed.
Hope that helps,