Tack,
I'm not certain voipms will allow two registrations to their network at the same time. However you can create sub accounts in voipms and then setup ring groups which will give you what you are trying to achieve. When someone calls your number the OBi and your softphone will ring at the same time if they are setup in the same ring group. You can even call each sub account as an extension number. Calling another extension is a free call on the voipms network.