The T-Mobile thing provides some of the advantages of dual SIM. But not all of them.
If you have a dual SIM phone with T-Mobile and some other MNO, if T-Mobile goes down you can still make calls (and use data if you have data plans for both SIMs). With this scheme, if T-Mobile goes down then you're fucked. With dual SIM if you're in an area that T-Mobile doesn't cover you can still make calls through the other MNO. With this scheme if you have no T-Mobile coverage you're fucked.
Which is why I have a dual-SIM phone with EE and Three. One dies, I can still use the other. If I'm out of coverage of one, I can still use the other.
Well, that's what I thought until they did engineering work on my nearest mast. Which took out both EE and Three for a few hours. Sometimes mast sharing has disadvantages...