My lil bro used to play. Our experience is that it is actually more difficult because we would both be fighting to get online, build our accounts, do our bit in alliances, wars etc. Imagine farming the ascended super weapon dmu back in '07 having to swich between accounts to make things 'fair' for the both of us.
I would guess that is the experience of most brothers (and sisters for that matter) who share a PC. Constant fighting over farm targets, raid targets etc.
Fun times
It is not difficult for admin to check for brothers playing against the rules though. Goods transferred from Brother A to a 3rd pty and back to Brother B is a big no no.