I'm not a MIDI expert, and I'm not following that exactly... Do you want one MIDI controller and two MIDI instruments? Or two controllers and one instrument?
The original serial MIDI was designed so you could daisy-chain multiple instruments and use one controller to address each instrument.
There's sort-of a general electronics rule that you can't connect to or more outputs together* but you generally can connect two or more inputs together.
If you want to quickly switch between outputs (two controllers into one instrument) you can use a [u]multiplexer[/u]. (The analog inputs on the Arduino ATmega chip are multiplexed into one analog-to-digital converter.)
- There is something called an open-collector or wired-or connection that does allow 'safe' connection of multiple outputs, but you still have to be careful to use only one output at a time so the data doesn't conflict.