you have DC with marking 40% and positioning 200%. both are white skills. if you train him using hold the line both attributes train equal amount and as if they were 120%.

you have AMC with marking 40% and positioning 200%. both are grey skills. using hold the line both attributes train as 120% and gain as much as with the DC (if we ignore age, fast trainer etc.)

MR with 40% and 200%. positioning is white skill and marking grey. using hold the line the player will gain as much as the DC and AMC but the skill points are not distributed equally. the white skill will gain faster.