AirPort connectivity

I assume that you want to use a Wi-Fi connection between the Time Capsule and AirPort Extreme, correct?

In that case, sure. Set up the Time Capsule and then manually turn off the 2.4 GHz signal on the Time Capsule.

Then set up AirPort Extreme normally. The default setup for the Extreme will be 5/2, so all that you will have to do is set up the AirPort Extreme the normal way to "Extend a wireless network".

Most Wi-Fi devices will connect to a 5/2 dual band network, but if you have some finicky Wi-Fi devices, they may not like the dual band 5/2 signal. You won't know until you try.

The security cameras, home theater controller, garage door opener here all have no trouble connecting to a dual band network, even though the manuals specify that the network must be 2.4 GHz.

Maybe ...

A few things to consider:

1. You are aware that Apple exited the networking hardware business back in 2018 ... right? That would mean, that most likely, you will find it more difficult to acquire new Apple routers going forward.
2. Depending on the exact models of each of these routers, they may or not provide both wireless frequencies simultaneously ... or allow you to disable one band or the other.
3. It will also depend on how these two routers will be interconnected (by wire or by wireless) to form the extended wireless network.

You can certainly set up the Time Capsule to broadcast 5 GHz only by turning off 2.4 GHz

The problems will come when you try to set up the AirPort Extreme for 2.4 GHz only. To do that, you will have to turn off the 5 GHz signal on the AirPort Extreme.

But......when you do that, the AirPort Extreme won't be able to connect to the 5 GHz signal from the Time Capsule.

You can set up the Time Capsule for 5 GHz only and the AirPort Extreme for 2.4 GHz only and things will work as you wish......but.....you will have to connect the Time Capsule and AirPort Extreme using a permanent wired Ethernet cable connection, or use powerline adapters if you can't run the Ethernet cable.

The latest models of each one are simultaneous dual band units and are capable of "wirelessly extending" their wireless networks. My networks are exclusively comprised of them in addition to older Apple AirPort devices. They are in fairly complex, mixed wired and wireless deployments and work well.

I'm not quite sure about separating 5 GHz from 2.4 GHz though and I question the need to do that. Even though each of those units can create separately named networks for each one, and those networks are bridged to one another in each unit, the benefit of receiving a 5 GHz signal but creating only 2.4 GHz as its output eludes me. I don't know if it's possible.