Sorry for the delay, I have somehow missed the notification about your response.
When configuring SSTP, the client side cannot be explicitly told to use a specific local address or interface. So my idea in this case would be to configure a second sstp-client interface, with the second user credentials, to connect to a different port than 443 (like 44443), use a firewall rule in the mangle table to assign one routing mark to packets towards server_ip:443 and another routing mark to packets towards server_ip:44443, then set up two routes to server_ip/32, each checking one of those routing marks, and finally another dst-nat rule in the output chain of the NAT table, changing the dst-port from 44443 back to 443.
However, this thread reveals that RouterOS still does not support the last step needed. So unless you can implement that step at the sstp server side or at some box-in-the-middle instead, I’m afraid there is no way currently.