Wait is this true? it's useless to have more than 1 brigade of armor in any division?
Not "useless", but too expensive for the little bit of benefit it provides. MOT is substantially cheaper both to build and to supply, yet has similar SA and only a little less HA than Armor. As long as you have one Armor brigade to provide its Piercing and Armor values to the entire division (only the highest brigade counts), the only benefit to a second armor brigade (aside from lower Manpower cost) is slightly reduced Softness and some extra HA. Combining ARM, MOT, and AC or ENG provides 3 different types of units for a better Combined Arms bonus. Early on, I'll use ARM+2xMOT+AC, but later I can build ARM+MOT+SPART+AC, which provides even higher SA, lower frontage, and a better CA bonus, but is a bit less resilient.
Late in the war, Manpower may become an issue, so using more than one MOT or MEC per division may not be ideal. At that point, HARM+ARM+MOT+AC or SPART may be a viable approach, where the HARM is providing the Piercing and Armor, and the second armor brigade is being used only as a lower-manpower source of HA and SA. In such cases, I'll spin off one MOT from each existing division and replace it with SPART or ARM to conserve manpower, then add those previously built MOT brigades to newly created armor divisions built without their usual MOT.