So, back on topic.
Difficulty is 20 (25 if you don't have the double-bladed specialization).
+5 to all parry rolls -or- +5 to one attack roll, decided on a round-by-round basis.
OR
+5 to all parry rolls -or- +5 to all attack rolls, decided on a round-by-round basis
Option one basically gives you a free parry or free attack, but subsequent attacks become harder. Option 2 gives you a free parry or a free attack, and the difficulty of successive attacks goes up linearly.
Thoughts?
I'm leaning towards option 2.
And a reminder that this would apply to any double weapon, such as electrostaves, force pikes and the like, not just lightsabers.