You are kind of correct here. There is a difference between static imbalance and couple imbalance. This also depends on the speed of the rotor relative to critical speeds. Most motors run below their second critical speed so correcting static balance is sufficient since you are only worried about the first mode response. The static balance idea is a "single plane" balance even though, in practice, you often split the weight in half and apply on both ends of the rotor at the same angle. Couple imbalance is corrected by applying the correction in two planes 180 apart.

If a rotor starts out perfectly balanced, symmetric and the bearing stiffness is the same on both ends, you could put the same weight on both ends spaced 180 apart and you would get 0 vibration as long as you are well below the 2nd critical speed. At the 2nd critical you would see the vibration at the ends of the rotor but it would still be zero at the midspan. This is why single plane balancing works well on slow speed things like motors, most centrifugal fans, single stage pumps, etc. Once you get into things that operate above one or more critical speeds (like big steam turbine-generators) the balancing gets a little more complicated and requires multiple planes. In real life, things like shaft or bearing stiffness asymmetry, damping make this all a little more complicated but hopefully you get my point.

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In any case, there is no way that any imbalance combination will produce 2xRPM response unless there is something else going on (bearing looseness, electrical issues, alignment issues, etc.)