Is this allowed? It's obviously the same person leeching his own accounts.

I never tried opening 2 clients at same time o-o;; Maybe it's possible? But if it's not and she/he did something to do multi-client/edited game files to obtain advantage over systems of the game. Then yeah that's not allowed, but if she/he have like spare laptops/PC's then I think that's clear.

It's definitely the same person, and they definitely use the same machine for all their accounts. I was farming in the sewers, seren and 4 others all popped into the channel next to me and I was like "hm that's weird". Then the 4 alts ran in to die in a pile to the monsters I was fighting. "Hm that's really weird". Then seren starts attacking all the monsters in my spot that I was gathering to AOE. "Wow how freaking rude".
They aren't doing a very good job either, dying left and right and using soul eggs to get back up again to keep trying to take over my spot. I make a comment about them using so many eggs and they say it's hard not to die a lot when they're getting lag from multi boxing 5 windows. He leaves all his alts in a pile on the floor to leech EXP from his main account while they're dead. Hold on, I took a screenshot of it even.