Question about Power Leveling

2 weeks ago my brother and I bought Guild Wars Prophecies and share one account. Recently, we decided that we want to play together and are willing to buy another game.

I have a 20 Mo/W and he has a 13 W/N.

Naturally, he has opted to restart his character under the condition that I will be able to power level him.

I was wondering how I go about this? Does the game have any restrictions about this? Will he get XP if I go and hunt with him in Old Ascalon, Regent Valley, Shiverpeaks, etc...

I'll be using my monk, and he'll be making a new W/Mo - so the two of us should be fine until we get to the higher level areas I presume.

I also plan on having his character use the double-xp rare scrolls. This won't be affected by me being in the party, will it?

exp is divided by the size of your party and level of the monster you're killing.
let's say your brother is level 5 and he kills a level 5 monster he gets 100 exp (if he's alone in the party). If you're there to help him kill it he will get 50 exp, you will get 0. if you're using henchies too, party size 4, he will get 25 exp.
if you and your brother are playing alone, 2 man party, killing monsters equal to his level and using scrolls he should be getting 100 exp per kill, kill higher level monsters and he'll get more
hope that helps a bit.
and here's some more deep going info ^^

easiest and fastest is yet take him from asclon to all deser missions,rush him though them and make him kill doppleganger.that 50k xp boost will take u for instance form level 12 to 18.(if u do normaly all pre searing quests+ the 3 starter missions in post+the 3desert you should be 12.).

from 18/20 simply do easy big xp quests like galrath or desert profession quest+15atrib ones.

lets say u make level 20 in 3-4 hours like that or less....faster than spend x time farming trolls or killing random stuff with scrolls.