This means that I won't be able to set the exta-sensor registers. As soon as I enable the MUX and attempt to write to the registers, the extra-sensor's registers would be overwritten?

If setting the channel control register to "no channel selected", wouldn't I able to read the extra-sensor registers while ignoring the sensors connected to the multiplexer? The 'i2c_select.disable()' function sets the control register to 0 which should be a "no channel selected" command.

I think I would need to work with a small footprint connector on the application board and find a way to make a flat-type connector which can also be connected to the programming board. I will try to contact with manufacturers to see if they can give some advice.

@MichaelKaras, The idea was interface both boards and control which programming interface to use from the Debug board using switches. Tag-connect would make things a bit simpler and rid the use of the Debug board but would also add the cost of providing the Tag-connect to the S/W engineers

@DerStrom, You are right, I'm looking for the most space efficient way to interface the boards. Without making a mess!