As Tom mentioned, if you are using a MiCollab server then the users and the devices should be configured on the MiCollab server through "Users and Services" rather than the MiVoice Business.
When you create the user through here you get the option to add the second device and it can create the multi-device user group automatically for you. It will also add the devices in there for you.
Normally if you have two devices, I would setup the first device with an extension number and it would automatically derive the second for you, e.g.
1000 for deskphone
1*000 for softphone