[PK]

Ivalero is right. Consider the following, where

F = Form skill
T = Technique skill
X = The result of a single 3d6 roll
Y = The result of a different 3d6 roll
Z = The result of rolling 6d6

and understand that

(X+Y) = Z, because rolling six dice together is no different than rolling three dice twice and summing it

Given the above, your system adds the margin from two rolls, so:

MOS = (F - X) + (T - Y)

MOS = F - X + T - Y

MOS = F + T - X - Y

MOS = (F + T) - (X + Y)

MOS = (F + T) - Z

See, Qhaysh, what you're missing is that you are rolling 6d6 for this spellcasting -- you're just doing it in two batches, but it's still effectively "one roll" because you're summing the margins.