It's all to do with vector forces, you need to split the forces on the coin into their component parts, each allomancer is pushing towards the coin, the line of force between the allomancer and the coin makes some angle relative to the ground, now as long as this angle is not 0 (eg. as long as the coin does not lie directly even with the allomancers but instead is slightly above or below their center of gravity) then these forces will have a vertical component in addition to the horizontal component, now assuming the allomancers are of similar strength, then their forces will be equal in magnitude and the horizontal components will be opposite in direction, the horizontal components will then cancel out, leaving only the vertical components to be considered. If the vertical component is downwards (The coin is below their centre of mass) then the coin will be pushed downwards by both the force of gravity and the allomancers, falling faster than normal. If the vertical component is upwards (The coin is above their centre of mass) then the coin will be pushed downwards by gravity but upwards by the allomancers, if this upwards component is equal and opposite to the force of gravity then the coin will be suspended.