1) My major uploads/downloads would not slow down her business and vice versa.
2) My investigation of NSFW websites would never be traced back to her business (although I know there a better ways around that).
3) If her network was down, I could run a cat-5 to her switch from mine and change her gateway machine setting temporarily and she was back up, and vice-versa.
4) If (slim chance) either of the firewall machines were to be hacked, presumably the other would remain un-hacked, and available.
Anyway, it worked for us.
Fundamentally, there may be no basis for anything.