Monitoring serial communication and problems with my RS232 listening device
The resistor and diode combination could be replaced with two diodes but then you would need an additional resistor pulling the output down to a negative voltage.

The diodes can only pull the signal positive, not negative.  RS232 operates between at least +5V and -5v (can be up to +/-15v) 

By using the resistor instead of a diode it is possible to steal the negative supply from the other input; the resistor also acts as the path for the signal when that input is active.

The resistor is not critical - it has to be much lower than the input resistance of the receiver it is connected to but must not be so low that is loads down the signal it is monitoring.

A few kilohms is appropriate - I used 4.7k when I came up with an identical circuit 20 years ago to allow two terminals to be connected to the console input of a voicemail system.

kevin