Since there are 6 human groups on Crestar, some character classes/combos may not have all character classes available. Some live above the tree line, so druid character class may not be available, as an example.
Human multi-class is slightly different that of other character classes multi-classes.
Humans must do one level of the first class listed below, gaining second level. Then the second character class must be learned, and advance to second level. No adventuring in the first character class, until second level is reached in the second character class. When both classes are at second level, then human multi-class advances like other multi-class character races. The human must meet the stat and alignment requirements for both classes. Once both character classes are second level, then human multi-class advances the same as non-human multi-class characters. See the AD&D 1E PHB for details.
After reading a long discussion on Dragon’s Foot forums, I decided that one game month of training for the second multi-class for humans would be how I am ruling on this. This presumes the player character will be using some part of their first character class training time learning about the second character class. Therefore one game week will be added to training times for the first character class.