By the way. And from linux you can mount the Windows partition, and from Windows you can access the linux partition as if they were your own disks or files.

Good answers both. I used duo boot, triple boot in the past, and Windows always wanted to be the dominant OS on installations. The OS that is installed last is the one that is going to boot first when the PC is turned on.

There is no problem. During Linux installation you can do all the operations you want on partitions and allocate space for everything. Format the one you want as common storage as ntfs and the Linux one as ext4. Just make sure you get the partitions right during the Linux installation, it's a very easy but potentially dangerous operation. Remember that if you do this installation you must set your BIOS to set the external disk as the primary boot and the internal disk as the secondary.

But what good is it going to do you, it's normal for operating systems to be on the computer's hard drive, not the external one.

Yes, if you can, make the partition with a tool inside base OS.

Also, if you use Windows you can install an application to read linux EXT2 format, and if you use linux, you can natively read NTFS. As another answer mentions, explore the option of using VMs