You need either an IP exemption, or a second IP address for your friend.
The former is the "best" option, as it will let you both play together with no other special tech required, and it will work forever (in case your friend comes back).
The one downside to them however is that they have to be approved by the staff, and it can take awhile to get one. In other words, stop reading now and go apply
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If you can't get one, another options (100% allowed by the rules) is to get another IP address some other way, for instance by having your friend get tethered internet from their phone, or by having one of you use a VPN (they cost like $5 a month, and let you use Bit Torrent without your ISP knowing).
But (and this is critical), whether you get an exemption or use a technical method,
you can never have one human play two accounts at once. If you so much as use your friend's laptop to heal yourself once while he's in the bathroom, you will be in violation of the server rules (and the punishment is permanent removal of both your accounts).