I have the ALT1000 transmitter and it works Great and I’d like to add a second unit. Can I have multiple external antennas and what are the precautions I need to follow.

It is best to use one common antenna with multiple transmitters. The RF output from the ALT1000’s can be fed to a combiner with a minimum of 20 dB of isolation between ports, such as the 100-3613 4-way splitter.