If you wanted to build a midi merger to combine two midi controllers to one output, but the two controllers will never be playing at the same time, could you just wire one midi jack to be split to two other midi jacks?
Not if they're both plugged in at the same time.
One will interfere with the other because MIDI uses a system where both pins are high and one gets sent low to start the MIDI message. The one that is not sending will hold the pin high. You'll also be shorting out that pin as the message gets sent.
Yes, also if both send a message (string of 1s and 0s) at the same time it will botch both messages, you basically need a little processor to act like a traffic signal.
Midi Solutions is the only company that makes a tiny, 2 in, 1 out merge box, I believe, which is strange, as most electronic musicians will need something like that at some point.
Some well designed controllers will have the option to merge the midi in signal with the local midi and send the combination out of the midi out jack.