Levelingup is a little different with multiclass characters because
you have to divide the Basic HP's and the CON bonus by the number of classes
each time you levelup. One way to conceptualize how this is done
in the game is with the following example. This example shows the steps
used to calculate hit points as a Fighter/Thief advances one level as Fighter,
then one level as Thief:

CHARACTER LEVELSUP AS FIGHTER. Take the result of the Basic Hit
Dice roll for Fighter (Table1) and divide by the number of classes, round
down and discard the remainder (e.g., 1d10 rolls 7, 7/2 = 3.5, round down
to 3.0).

Take the Constitution Bonus and divide by the number of classes, add any
CON remainder from previous levelup, round down but KEEP the remainder
(e.g., CON bonus of 3, divide by 2, 3/2 = 1.5, add any remainder from previous
levelup, +0, round down to 1.0, keep the CON 0.5 remainder for the next
levelup).

Add the Basic roll results to the rounded CON bonus to get the number of
hit points awarded to the character (e.g, 3.0 + 1.0 = 4.0).
The Fighter would go up 4 HP this levelup.

THE CHARACTER LATER LEVELSUP AS THIEF. Take the result of the Basic
Hit Dice roll for Thief (Table1) and divide by the number of classes,
round down and discard the remainder (e.g., 1d6 rolls 6, 6/2 = 3.0, round
down to 3.0).

Take the Constitution Bonus and divide by the number of classes, add any
CON remainder from previous levelup, round down but KEEP the remainder,(e.g.,
CON bonus of 3, divide by 2, 3/2 = 1.5, add the remainder, 0.5, from previous
levelup (step 2), 1.5 + 0.5 = 2.0, round down to 2.0, keep the remainder,
0, for the next levelup).

Add the Basic roll results to the rounded CON bonus to get the number of
hit points awarded to the character (e.g., 3.0 + 2.0 = 5). The Thief
would go up 5 HP this levelup and there would be no carry over CON remainder
for the next levelup.
A similar method would be used for a triple class character, except that
the Basic Hit Dice roll and the CON bonus would each be divided by 3 (the
number of classes).
In BG a doubleclassed character gets the extra HP (from the fractionl
CON HP bonuses which are retained and summed) every other levelup.
A tripleclassed character gets the extra HP every third levelup. 