Say I had a monophonic modular analog synth patched. If I played two or more notes, would it mess it up or would the MIDI to CV module just send one note through?
In a monophonic MIDI to CV module it would just send one note through.
It is called note priority, and the implementation depends on the MIDI to CV module. Some modules will let you switch between note priority modes (like this one), some implement one in particular.
Some modes are:
Low note priority: The lowest note (lowest MIDI value, lowest frequency, lowest control voltage) will have priority over the others.
High note priority: The highest note (highest MIDI value, highest frequency, highest control voltage) will have priority over the others.
Last note priority: Last note played will have priority over the others.
If you press C2 and E2 exactly at the same time, in high note priority E2 would pass through, and in low note priority C2 would pass through.
Where documented, it is often written that the lowest note will be played but this may be model dependent and in some synthesizers even switcheable. If you have arpeggiator and turned it on, the synthesizer will play all notes of the chord in a sequence.