I hope someone here will be able to help.

Can one safely connect two devices that would read (Rx) from Cambridge
302 (Tx)?

I have a signal right now going to ClearNav only but I would like to
split the Cambridge out signal (Tx) so I can also receive the data in
a backup PDA. The second device would only receive the data from
Cambridge not send anything.

I found some references that suggest this is commonly done with RS232
interfaces, but I am a bit suspicious since connecting 2 devices would
reduce resistance of the load. The manual says two PDAs are not
supported but it is difficult to say if this is dictated by circuit
issues or by protocol being messed up by two PDAs trying to
communicate bi-bidirectionally at the same time. Again, I only want to
receive.

Any knowledgeable reply would be appreciated.

It is also interesting to see PowerFlarm specifying that only one
external device could be connected to the DB9 serial port. I would
like to understand that as well. What is the reason for this, hardware
or software?