Yes, one serial transmitter (TX) pin can drive several receiver (RX) pins when serial ports are used in our avionics equipment. As far as I know, there is no real "spec" max loads, but several should be no problem. However, wiring two or more TX pins together will cause massive data errors and will not work. And it is not unusual for a port to have several different other ports connected to it as long as the above guidelines are followed.
The "real" RS232 spec addresses two devices connected with a standard serial cable with standard connectors with several additional signals to control the connection. This is not fully implemented on our equipment which thus allows the multiple receiver per transmitter operation between multiple devices.
Serial data streams are not really "DC" but pulse trains where the voltage levels swing rapidly between two values, that can vary from as much as -12v to +12v, although more commonly are just several volts in both directions and for some gps equipment only swings from ~0v to ~3v. The serial data rate is usually required to be properly set on both ends to allow valid data to be extracted from the bit stream.
