I think this might have been a bug that was FIXED a long time ago. As with most quest rewards it's a straight award of XP and not like XP gained from killing a monster.
In short, no it won't work, but go ahead and try it. It's like 1k and 1 minute of your time.
You can use it while fighting, but the EXP scroll would have no use, BUT you can farm the doppel for drops.
he does not drop anything, at all.
he used to, waaay back, and everyone was farming him for yellow/purple weapons. the Anet fixed it and now i don't think he drops nything, and if he does it will be a normal white thing
And actually, every time you kill him you get 100XP so if you reeeeally wanted to waste scrolls like that... I've been farming him recently just for kicks, because it's funny to see him die when he's suposedly my superior mirror image...
The scroll will work on the exp you get from killing him, usually 100xp if you are level 20, but will not work on the 50k exp you get as a reward of ascension. BTW, I don't think I ever earned the 1000xp from the bonus although I'm sure I killed him in well less than 50 seconds several times.