Dual SSD / RAID1 in a1409 Time Capsule?

Theoretically, maybe. Keep in mind that the A1409 TC was discontinued 12 years ago; the Startech unit was also discontinued some years ago. You have aged equipment here. If you want to experiment, fine, but I would not rely on it for any critical data or for backup.

The Startech unit appears to present itself as a single SATA drive to the "external world" meaning the host, in this case your TC, so it would appear to be possible to use it inside a TC. The RAID0/1/JBOD setup is a hardware switch in the Startech unit.

But I do not see how RAID1 would result in any performance improvement in this setup, and it certainly does not give you any multiple-drive capacity. The SATA interface is a choke point, as well as the WiFi speeds. And it certainly does not provide any kind of hardware data protection.

I’d scrounge hardware for a TrueNAS or similar configuration with Time Machine server, or scrounge an existing analog to a Synology NAS or Ubiquiti UniFi UNAS Pro (with Time Machine server) and preferably something with more easily accessible bays.

This as the TCs I was dealing with got old, got flaky, and got replaced.

What the TC might do with any changed commands, or changed responses the RAID-1 might send back?

If you do decide to experiment with your hardware here, you’ll probably want to create some way to see that status LED from outside the TC chassis.

Might be easier to 3D print a new chassis for the TC too, if you wanted to invest the time and effort in this.

I think that Martin nailed it on the logistics.

It seems like a waste to put two SSDs into the old TC. If I was upgrading the old TC, I would just use a larger 3.5" HDD or perhaps two larger 2.5" HDD's in the Startech adapter if it fits.

Then I would use the two SSDs in faster individual USB 3 enclosures or in a USB 3 dual dock and use them for local redundant clone backups.