"You see this."

Once, I defended splitting hit die in 1E and OSE-AF, and said something like, “A F2/MU2 must have two hit die because they are 2nd level!” And I would have backed that up with something like, “If you plan to roll a hit die each time a class goes up, then you’d also have to roll one for each class at 1st level—so a F/T/MU could end up with 23 hit points at 1st level! (10,6,4,3 with Con in 1E, or 19 in OSEAF, 8,4,4,3 respectively.)

The cart is firmly before the horse, there:

You could totally start with one split HD and then just add one appropriate HD as each class goes up. You gained a whole class worth of XP, why shouldn’t you gain a whole HD? It’s like the game is trying to protect you from yourself because you would choose to attack and save as a 2nd level PC even though you have 4HD, and it does this by making your HP worse.

Let’s also consider the brutality of requiring a multi-class PC to carry FULL XP loads for each class, which means they’re paying for features that are redundant or even inferior to the others they carry. A point that belabors the inevitable: The multiclass PC will only ever be 1-2 levels behind the prodigous Thief.

Let’s take a moment to dwell on the fact that such a player could opt to roll up 2 or 3 whole entire PCs and play all of them with greater potency, enjoy more attacks per round and a greater pool of hit points.

And let’s not forget to look hard at the fact that a multiclass PC endlessly dumps XP into a class after they reach one of their level caps.