the only way to play 2 or more id-network or merge-network sites at the same time is to have 2 or more ip-adresses at your home.

This is the only way to have more than 1 open at the same time
But be carefull not to play in the same games or tourneys from 2 different network sites at the time, they will check all ip adresses and the home adresses and if they find out you are playing the same game from 2 positions its considered cheating and they will freeze all your accounts on the network and confiscate all your funds and give you a live ban.
The only other possibility is to write to support of the network and explain that you want to use 2 of their sites at the same time for 2 different players that live at the same addres, they can make it possible for you and your partner to play even on the same site from 2 computers with 2 different accounts.
if they are willing to accommadate this you both have to underscribe a exclusivity paper that you will not play in the same games at any time, and this will be monitored strictly.
wolfie