Well thirqual notations are true, an "attack action" is an abstarctionof a series of blows and swings. But, RMSS uses a three-phases system and he wants to use this option only as a deliberate (possibily react & melee) attack. Like he parried all round committing attention to defense only and then landing a single solid blow with his two-handed weapon. By the rules, I know he cannot. But, try to imagine that, seems not so unlogical to do. The problem is, overkilling.
@ Peter: I wouldn't allow anyone to use a two-handed wapon with a normal shield. A normal shield is larger, and heavier, than a target shield. It must be handled and if the defender is not grasping the handle of the shield it does not stay in his place, but will hang down the shield arm, possibily hindering movement beyond reasonably being able to use shield arm to attack or manipulate anything. A buckler, as I intend it, is a much lighter and smalle shield, which is tied up to the vambrace like a large gauntlet. You actually MUST handle it to use it against a target and take the DB bonus, but you can let him loose and it will not hang down hindering movement too much. Still, a -20 penalty seems appropriate.
The question is: would you consider changing stance from 1 handed weapon & shield to 2 handed weapon an action by itself, and if yes, how much activity will it take?