I envisaged the same line to be used for straight and paddle keys. here is how :
1. the morse line is analog input pulled high by an external resistor of 4.7K from 5 volts.
2. the straight key is connected normally. the paddle is connected such that the dot switch has a 4.7K resistor in series and the the dash switch has a 10K resistor in series
3. when the straight key is pressed, the analog line dips to below 0.5v
4. when the dot is pressed, the 4.7K resistor shorts to ground, the voltage at analog input is 2.5v or thereabouts.
5. when the dash is pressed, the 10K resistor shorts to ground, the voltage at analog input is now 3.75 or so.
6. when both the paddles are pressed (Iambic), then the voltage is 1.75
This is the keyer logic that i am building on ubitx too.