First of all compliments for this wonderful retroclone that mixes the simplicity of the Original RPG with the best features of the 3.5.
After having read the rules I thought about a system to build combined class freely and not only for the Elves.
These are the rules:
1) General Rules
(Quoted from Basic Fantasy RPG rules version 1.07)Combination
class characters use the best attack bonus and the best
saving throw values of their original two classes, but must
gain experience equal to the combined requirements of
both base classes to advance in levels
2) Allowed Classes
Characters can combine max 2 classes,and only classes their race is allowed to choose.
(So a Dwarf can become a Fighter/Cleric but not a Magic-User/Fighter)
I've got two ways for that,you choose which is the best way for you
1) Two dices
Roll both hit dice for each class,sum the result and divide it for 2
2) One Dice
If both the classes have the same hit dice just keep it.
If not,sum the faces of the hit dices of both class and divide the result by 2,if the result is an odd number round down.
(A Fighter/Cleric does (d8+d6):2=7,but while there is not a d7 round down to d6)
(A Cleric/Magic User does (d6+d4):2=5,but while there is not a d5 round down to d4)
By the way,reading the rules I didn't get how the hit points for the combined classes work after 9 level,do they get the better bonus to hit points between the two classes?
I hope you will find these homemade rules interesting for you campaigns.