The other system which I seem to have on my VW passat is derived from the antilock braking sensor, and it works on the principle that when travelling in a straight line, both wheels on the same axle should rotate at the same speed. If there is a difference it means one of the wheels radius has changed, and this likely due to a change in pressure. with this system it does not measure the pressure so as long as both wheels on the axle have the same pressure within a reasonable margin it should be happy. So far (4 years in) no problems.
I don't think this is quite complete, in that I thought the VW system does accommodate different rotational speeds between wheels to correct for tyre wear?
So they don't have to rotate at the same speed, just rotate at the now expected correct differential speeds, clearly also on running for long enough to average out cornering effects.
Our Mk 7 Golf's I have had to shall we say "synchronised" for wear by accepting a warning after I had confidently confirmed to myself was only due to wear, this was directly after I change my wearing fronts to the rear.
I have had another warning, this was when the rear seats had two passengers. That's, a very very rare event as we only normally run either solo, or just the two of us up front. Simply that extra tyre loading affected the expected rotation speeds front to back.
Assumed a problem till I checked the pressures manually, then simply completed the few mile lift.
A further time I had a warning was coming back from a Halfords complete tyre replacement. There the pressure were all different to each other, way off right from under to over inflated.