You wouldn't do it that way. You'd keep them both, then cut or trade Dak in 2024. Remember, any extension Murray signs doesn't start until 2024. He'll still be on a rookie deal until that time. But these are wild hypotheticals. No chance of it happening.
The base salaries don’t start, but the signing bonus proration will.
Dak is currently set to be a 20m$ cap hit this year and a 50m$ cap hit in 2023… 40m$ in dead cap if you cut/trade him in 2024. Kyler is a 12m$ cap hit this year, and a 30m$ cap hit in his year 5 option.
So let’s say they give him the same 4-year extension Dak just signed; that gives you control for 6 years. 4 years, 160m, 66m$ at signing prorated over the next 5 years. That’s 13m$ per year. 94m$ Of the total value remains, which you can try and wiggle around to avoid major cap hits the first two years you have Dak too. Let’s say you put 2m$ as a roster bonus in 2022 and 2023, then 45m$ salary in 2024 and 2025. That’d be reasonable.
This mean if Dak is cut in 2024, these are the combined cap hits for the two players:
2022: 47m$
2023: 95m$
2024: 97m$
2025: 58m$ (just Kyler)
You could try a June 1st for Dak in 2024, that should move the bulk of it to 2025, but then 2024 will still be 60+m$ and now 2025 is also approaching 100m$
impossible is a strong word because TECHNICALLY you could pay 1 player 150m$ a year and make it work, but implausible is a good word for this scenario. Restructuring Dak’s contract twice already has made any changes to the QB position for the foreseeable future unrealistic.