How To Link Two Computers Together Via Modem Locally:

This is actually easier to do than most people think; Just run a phone cord
between the two modems (using the LINE or WALL jacks, of course). Run a
terminal program on one computer, and issue the ATH1 and ATO commands. This
will make the modem go immediately into an on-line state. Now run a terminal
program on the other computer, and type ATA. This computer will make a modem
"answer" sound, which will cause the two modems to begin communicating.
That's all there is to it.

If you want to join two computers directly without the luxury of being able
to issue commands directly to the modems in question (and the program you are
using will want to dial the modem to reach the other computer), then you have
a trickier problem. The problem is that the modem will expect a dial tone,
and immediately cancel the operation when it does not detect one. This is
still not too hard to deal with, however. Begin by plugging the dialing modem
into a real phone line, as usual, and have it dial some number (any dummy
number will do, just something to make it think it has dialed some number).
As soon as it has begun dialing, unplug the modem from the wall, and plug it
into the answering computer. Then have the answering computer answer the
line, and the two modems will think that they have connected over a real
phone line instead of directly through a phone cord. That's all you need to
do. Alternatively, you can add an X1 command to the initialization string of
the dialing computer's modem, which will prevent it from looking for a dial
tone before dialing.