Oddly, the only time this was even semi-officially addressed to my knowledge was in Dragon #243 in 1998 (and yeah, I know, that's well into 2e). Still, for those interested, this is what Skip had to say:
"A multi-classed character's abilities are subject to considerable debate, even at TSR, inc. Here are some recommendations and observations on the matter:
For hit points, roll all the PC's Hit Dice, adjust each die for Constitution, and note the total. A character's hit-point rating is the average of all the dice (the total divided by the character's number of classes). Drop the fractions, but don't discard the total (you'll use it to help determine the character's hit points as she gains levels). Each time the character gains a level, roll the appropriate hit die, adjust for Constitution, and add it to the recorded total. Recalculate the character's hit points by dividing the new total by the character's number of classes, again dropping any fractions."