Need Advice on dual Boot Win 11 Pro and Win 10 Pro Computer

About ready to take the plunge. Dell XPS 8930 tower computer has retrofitted I-9 9th Gen K-series CPU which is max available for existing socket/MoBo, so computer "should" also have TPM2--I hope. All other physical specs are consonant with Win 11 Pro. I understand that it's best to adjust BIOS settings (UEFI, AHCI, etc.) identically for both Win 10 Pro and Win 11 Pro OS. From reading, it appears that installing Win 11 Pro first is mandatory. Please advise if I'm wrong about anything.

In order to save money and some hassle, is it possible to D/L trusted and free versions of both Win 11 Pro and Win 10 Pro, from trusted sites? Please, which specific D/L sites are GTG? I understand I'll need an Activation Code (correct terminology?) for each OS. Where are trusted places to obtain both codes? Obviously, I'd prefer above-board sites like Amazon or even Ebay for obvious reasons.

Due to personal time constraints, and my personal ignorance, I'll be having a trusted computer Pro wipe and partition existing 2TB SSD, I/2 for each OS. Instructions on accessing a given (Win 11/Win 10) OS will be appreciated. Hopefully such a dual boot system will automatically give me a choice on start-up.

TYVM for comments.

Before doing anything CREATE A SYSTEM IMAGE.
There are many step by step guides and videos available as well as several ways to set up dual boot.
One such is here

Rather than dual booting, my personal preference is to use a virtual machine for the second OS.

Many folks here download their isos from UUP UUP dump

Why waste money? Just shrink your current partition, then install windows 11 on the unallocated space. Will keep your current OS intact and make your pc dual boot with 10/11.