No, the stack is not valid.
It comes down to a matter of weight. Whether or not we agree with Crawford regarding "suitable anatomy."
The rules on riding a mount specify that a mount must have a suitable
size and anatomy for you to ride it (PH, 198). If the game makes an
exception to part of that rule—about the creature's size, for
instance—the other part of the rule still stands.
The matter is actually settled elsewhere in the rules. Carrying capacity, whether you use variant encumbrance or not, is still limited. So even if we disagree with Crawford, and think that centaurs have the right anatomy to carry other centaurs, we are still left with the issue of how much they can carry regardless of its shape.
Certainly, a particularly small centaur may be light enough for a particularly strong centaur to carry on their back. I would not be surprised if parents do this with their children regularly, and I would not be surprised to see one centaur carrying an injured comrade in this manor.
But player characters are typically adults, and we are talking about riding a mount, not being carried along due to injury. So a given, especially strong (str 20), PC centaur may be able to carry 600lbs.
Carrying Capacity. Your carrying capacity is your Strength score
multiplied by 15. This is the weight (in pounds) that you can carry,
which is high enough that most characters don't usually have to worry
Equine Build. You count as one size larger
when determining your carrying capacity and
the weight you can push or drag
Size and Strength. Larger creatures can bear more weight, whereas Tiny
creatures can carry less. For each size category above Medium, double
the creature's carrying capacity and the amount it can push, drag, or
lift. For a Tiny creature, halve these weights.
A given PC centaur might possibly weigh less than that, depending on how "medium" PC centaurs really are. Therefore, it is possible for one centaur to act as a mount for another. It's possible to lift up to double your carrying capacity though, with limitations.
Push, Drag, or Lift. You can push, drag, or lift a weight in pounds up
to twice your carrying capacity (or 30 times your Strength score).
While pushing or dragging weight in excess of your carrying capacity,
your speed drops to 5 feet.
So one additional centaur could be "lifted", but would reduce the speed of the bottom centaur to 5ft. Beyond that, the rule says "up to twice your carrying capacity." Which means that beyond that, beyond 1200lbs, it's impossible for the centaur to move the weight.
That is assuming that your DM even allows you to "push, drag, or lift" by carrying something on your back, which is highly questionable. Otherwise, you are stuck with the 600lb carrying capacity, and that's it.
So we can conclude that a centaur can carry another, if the DM decides that the anatomy is acceptable (personally, I don't see a problem). But they need to be a strong centaur, and/or the other one must be particularly small. Beyond a single rider, movement of the "stack" is reduced to 5ft if possible at all, and it's nearly impossible for there to be more than two centaurs being carried, unless they are children.
If you use variant encumbrance, even one centaur rider is going to cause encumbrance, probably heavy encumbrance - so movement will be reduced by 10-20 ft regardless, and you may:
...have disadvantage on ability checks, attack rolls, and saving throws
that use Strength, Dexterity, or Constitution.
So just one rider that heavy is a massive liability, the "stack" would make everything worse. And don't forget, while the movement penalty only matters for the first centaur, all the others suffer from the disadvantage except for the one at the top. Meaning they are far less effective in combat than the same number of centaurs all moving independantly.