I have never heard of a manual transmission that had this feature. It's possible that some twin-clutch boxes could, but that's way too new and expensive for me to think about.

Thrust would have to be taken into consideration, as it would be reversed and there may not be thrust provisions in sides that normally don't get thrust. I'm thinking about the countershaft.
Then there may be problems with oiling hole orientation and if the transmission has its own little oil pump inside (most don't, but I believe Gear Vendors overdrives do), it'd pump backwards.

What is this, create a V8 out of two Honda 4s?

R.