TTL logical low isn't ground
If I understand your problem statement correctly, you are saying that when you try to transmit a logical low from at lest one device to the Due, the level converter only manages to pull the Due RX wire down to 2 Volts?

What does the RX wire look like on the other end (5V end) of the converter?

I would expect something like this to happen if the sensors are using too strong pull-ups, or if they are not intended for fan-in, and thus all try to transmit a logical high when one of them is trying to transmit a logical low.

If that's the problem, I would attach each of the sensors with a diode, Cathode towards the sensors, such that the sensors can only pull the line down, not up. Then I would use a pull-up on the common end (anodes) of the diodes, which you connect to the level translator input. Perhaps a 2.2 kOhm pull-up would be appropriate (giving about 2.5 mA current through the diode when pulled low.)

Also, did you read the top paragraph of the description you linked to? The part that says:


  Sale: We've released a new rev of this Logic Level Converter that fixes the issue with the board not stepping down from 5V to 3.3V correctly. We've also dropped the price on this model and we will not be stocking this rev again once it's gone. Get it while you can!