I thought WoW Classic and Retail were "separate" clients?

[Kolanu-wyrmrest-accord]

Cross linking my thought process on why that is not as easy as it sounds.

Why can't we play Classic and Retail at the same time?

Speculating on why, Classic is built as a layer off of retail. They sort of Forked the client to allow both to work without having to completely install components of the game twice. Multiboxers run multiple clients from the same computer so the issue isn’t on the player end. However, the Login Servers are designed to look for an account logged in twice. Because of the way that the login servers work, it checks for a prior login and boots it. This is similar to the “Your account has logged i…