I am kind of confused as to why this is in the SIP category... The Superset 4025 is a proprietary digital set.
And your communication tech is correct... The 4025 is a single end-point and it cannot share the connection with another phone, and I am unaware of any Mitel system where you can make two phones have the same extension. That said, you could make appearances of each one on the other set, but it would consume to ports in the system. For example, one is x100 and the other is x101... you can program an appearance of x100 on a key on x101 and on x101 you can put an appearance of x100 on a key.
Of course, all this is dictated by the system in use... Which is likely going to old as the 4025 has been discontinued for well over 10 years.