Adding a second bootable drive initialise as MBR or GPT?

For Windows 11 there is only BIOS UEFI for supported installations.

That is UEFI native (without CSM)

For Windows 10 there were BIOS options for Legacy or UEFI native or UEFI hybrid (with CSM).

A BIOS that is Legacy will have a default Windows drive with a MBR partition.

A BIOS that is UEFI will have a default Windows drive with a GPT partition.

All other drives (non-Windows drives) can be MBR or GPT (storage drives).

A storage drive with MBR partition can be moved from a computer with BIOS Legacy to a computer with BIOS UEFI.

A storage drive with GPT partition can be moved from a computer with BIOS UEFI to a computer with BIOS Legacy.

If you want to have an additional Windows bootable drive it should be GPT.

The GPT drive by default will have an EFI FAT32 partition to boot with BIOS UEFI native (without CSM).

This is dual booting.

Power outages / surges can injure multiple drives.

One of the quickest methods to recover from a failed drive is to replace the drive and restore a backed up image.

Microsoft had deprecated backup imaging.

There are third party software that can be used for making backed up images.

(Acronis, Aoemi, EaseUS, Hasleo, Macrium, Paragon, etc.)

There are free and pay versions.

The backed up images can be saved to another drive or the cloud or both.

One or more of these links may be useful: