Magic trackpad starts pairing but does not complete succesfully with one Macbook I have two Macbook pro retina's:  


MbPro 15" retina late 2015 El Capitan
MbPro 15" retina mid 2012 El Capitan


The first one is able to pair with the Magic Trackpad (not the "2" - instead the older version).

But the 2012 one has the following behavior:


initially detects the trackpad -including showing the serial #
but then cannot complete the pairing - and I'm back to square one.


The initial detection of the trackpad happened several times. 



But usually I do not even get that far: the trackpad is simply not detected.

There is obviously no problem with signal or batteries (new): the trackpad works all day on the other MBPro.

What should I look for as potential culprits and fixes?
Just found a related question and answer: apparently the trackpad can only be paired with one mac at a time - even if the other one were powered off (as is the case here)

How can I pair my Magic Trackpad with multiple Macs?