Speed and reference sensors...
 

I just took them out, one by one, looking at the first one, checking it very carefully, and when I got out the second, I found that this sensors are the same, so I have a question, why do you have to be very careful about where they go?, being them the same sensor, the same part # 0 261 210 003, what would be the problem?, I put them back in the same place they were, but I got curious...

technically you can swap them as they are the same, what you do have to be carefull about is which socket you plug it into

The flywheel has two pins on it and each are in a different location. One sensor tells the computer the engine is turning (speed sensor) and therefore the DME should keep the fuel pump running, the other tells the DME the location of the crankshaft (reference sensor) and how to set the timing of the spark. If it is reading the location of the crankshaft incorrectly (happens when it's plugged into the wrong set of wires), the timing cannot be set properly and the engine will not run.

Re: Speed and reference sensors...
 

IF you have an early car (pre-'85/2), it will not be an issue at all.
IF you have a later car, the concern is NOT so much getting the same sensor back in the same hole, as much as it is making SURE you notice the shim that goes under which sensor to keep the spacing correct for such. That is the issue. I think. ;)

Note: even later cars, use a smaller TDC sensor also, and not 2each of the same. IIRC.

In other words, YYMV. :) Check it out very carefully, for YOUR car's model. They are NOT all created equal. Know this.

You, did NOT specify what year and model car your dealing with here.

Amp connectors ARE another concern, and their proper placement.

The sensors are the same. They are interchangable. What connector they plug into is NOT interchangable. If you get the plugs crossed, the car will not start.

AFJ

Hey Razor there is only one pin, the other sensor picks up the flywheel teeth.