I am not sure what you mean connect to both systems... it CAN read both firmware and will look for a MiNET one first, if not found from the FW server, it will look for a SIP firmware and use that.
What I can tell you is that the MiVO 400 requires a Mitel SIP Phone with SIP FW, not MiNET. I don't think that has changed.
I can send you a guide how we do it with the older batch of phones we had before if you need it.