There are 6 possibilities:
1. No doublestrike, no glancing blow
2. No doublestrike, glancing blow
3. Doublestrike, no glancing blows
4. Doublestrike, glancing blow on first but not second
5. Doublestrike, glancing blow on second but not first
6. Doublestrike, glancing blow on both.
From a quick test, I saw all of these possibilities occur (using an actual 2-handed weapon, though) except for 5.
So without more comprehensive testing, I think it's safe to guess that glancing blows occur on doublestrike just like they occur on the primary attack. Which makes them... no better than outlined above, where that result was assumed.
A more interesting question is whether you can automatic shield bash on doublestrike. I just assumed you could, but maybe you can't.