Your terminology is incorrect and that makes it difficult to explain this to you. There is no such pins like TX1, TX2, RX1 and RX2 oin a MAX232.
The correct pins are (pin numbers in square brackets):
To make a two-way spy/sniffer, you need to use two receivers (one MAX232 contains them both). Pins 8 and 13 connects to a "RS232"-level line each (TXD and RXD) and the sniffer logic connects to pins 9 and 12.
If you just want to listen in on a single line, just connect eg. pin 8 (R2IN) of the extra MAX232 to the "RS232"-level line that you wanna monitor and connect your sniffer circuits logic level to pin 9 (R2OUT).
You should never connect two transmitters to one line of course. It won't hurt anything (provided the EIA/TIA232 protocol has been adhered to by the equipment manufacturers), but the data stream will be like psychotic babble.
If you've got an old PC with a serial port, you can just use that with a sniffer program and hook up the RXD of the serial port to the line you want to monitor.
What protocol are you trying to hack?
Did you search for somebody who allready did the work, to save a lot of time - most protocols are hacked within days afer their release.