I did attempt this in Pathfinder and I got pretty close. Another individual really wanted it and finished my work and it has its own
GitHub Repository.
The idea is if you wanted a Fighter/Rogue you would add three character classes. First is the Fighter then a level of Rogue and finally a new Gestalt class. Basically the idea is the new Gestalt class handles merging of HP, Skill Points, saves and the characters true level. Then on the Gestalt Class tab you select Rogue/Fighter and both of those levels are "removed" from HL leaving the character as a level 1 Gestalt class.
This is all done so that the Rogue/Fighter tabs are still displayed allowing for archetypes or picking of class specific abilities. It will also allows for multiple spellcasting classes.
The above "works" but its not pretty. Its NOT easy to work with or understand the scripting but it works. I for all purposes is a 100% Hack of Pathfinder game system.
