The horn/compression driver in the BC112 MK3 is offset to one side of the cabinet, the port is in the other side. If you mirror the baffles, providing you stack the two cabs the right way, the two “tweeters” will be as close as possible and tend to act as one. The further apart they are, more frequencies will be subject to what is called “comb filtering” where some frequencies are amplified and some attenuated.
In the real world, it may not be a big deal but it’s certainly good practice.
The irony is that when I built mine, I fitted the baffle the wrong way round. Of course with a single cabinet it does not matter.