Not really a specific question, more a call for help. But anyway, because it's fun to think how to do things...
If you don't want to use GPS, then yes an accelerometer would be best. This would give you readings as to what type of movement A has done. You would then replicate the movements with B. The trouble with the accelerometer is that it has no idea of where A physically 'is' - just that it has moved in X direction for an amount of time. You could look at something simple like the XLoBorg from PiBorg (http://www.piborg.com/xloborg) if you didn't want to mess around too much - it's a GPIO plugin board that monitors motion and direction. You will potentially need accelerometer boards on both Pis so that B knows what ground it has covered (you could of course just take it's word for it).
You would then need a way of A communicating with B. Either do this over wifi or you can probably use something like http://shop.ciseco.co.uk/slice-of-radio-wireless-rf-transceiver-for-the-raspberry-pi/ to communicate wirelessly over serial. Either way you'll need to write the software to both send and receive.
There is complexity here that I don't think you can get away from and possibly some tricky maths if you're not careful.
The only other option is to get A to tell B what it has just done. So if A moves forward 5, turns left 90 degrees and moves forward 5 again, it could just tell B that information and B could replicate it. This would give a master-slave configuration which could be just the ticket if you needed simplicity.