If I have a formation with two 10,000t HQ units can it support a hierarchy of 20,000t?
No.
Redundant HQs reduce the chance of the officer getting killed when an HQ is destroyed.
They actually do not, and if you work out the math you will see that multiple HQs actually
very slightly increases the chance for a commander to be killed.
The way commander deaths work is if you have N headquarters units and one is destroyed, there is a 1/N random chance that the commander is killed. However, it is also N times more likely for a the HQ element to be targeted and thus hit in the first place, so the result is that the commander is killed at the same overall rate.
However, since these additional HQ units take up tonnage in your formation (assuming you built your formation up to the tonnage limit of your HQ), combat units must be replaced to make room for the additional HQ units. Since HQ units are usually marked as "Non-Combat Units" in the ground unit creation screen, this small reduction in "Combat Units" in the formation causes the HQ element to be targeted
slightly more than N times. It is a small effect but readily derived.
The benefit of multiple HQs is really that if one HQ unit is destroyed, it is possible to immediately replace the commander to preserve the formation command hierarchy (== bonuses passed down the chain) and provide combat bonuses to that formation and subordinates. However, usually it is best to place superior HQs in support or rear echelon formations where they are rarely targeted anyways, and the number of available commanders tends to be the biggest limitation on ground forces so there are not many spare commanders to use as replacements, if any. Because of this I usually don't consider having multiple HQ units to be very worthwhile, especially because the larger HQs tend to be overly expensive which can drive up the build cost of a formation significantly.
