Perhaps changing the MIDI-channel on them would help?
Seeing as atm only 1 will connect at a time, just connect 1 and set it to MIDI-channel 2. Connect the other and set it to 3 (or any other than 2 and "any"). The MIDI-channel should store internally, and perhaps help OSX identify them as seperate units.
Remember, any change here will affect how DAWs and MIDI hardware route signals to them, so they will require the correct corresponding MIDI-channel to be selected.
Otherwise perhaps, connecting 1, making a manual change to the MIDI devices name in OSX, and once done connect the other.
Mind you, this is just quick thoughts. I don't own 2 and haven't had similar issues, but think it's worth a shot.