i remember playing around with this before, being nosey. If you stack multiple IP's on the NIC and in SQL Server config manager you basically stop each instance from listening on all IP's which disables the IPAll config section on the TCPIP parameters. You then configure the IP you want to listen on for each instance and supply the same port number 1433, not wise but i do remember it worked. In SSMS studio i connected to the following instances
INST1 192.168.1.49,1433
INST2 192.168.1.55,1433
have a play with it and see how you get on
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"Ya can't make an omelette without breaking just a few eggs" 😉