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That's my point, this is not a Chromebook hence no Developer Mode to worry about so you need to partition the drive and boot to USB as you would to install any OS and load Linux and Chromium. I'm not 100% sure which would be better to load first as it sometimes is easier when one OS is loaded before the other. How you'll edit your bootloader is also going to be a thing you'll need to address so you'll have both listed as bootable options.

If it were a Chromebook then you'd be in business.

https://www.chromium.org/chromium-os/developer-information-for-chrome-os-devices/generic

Introduction

Enabling Developer mode (dev mode) is the first step to tinkering with your Chromebook.

Note the last word... Chromebook. I may be wrong (doubting that but possible) but you are using a Windows laptop as previously stated, you do not need to enter Developer Mode except when trying to load an OS that is not ChromeOS on a Chromebook. You're just dual booting a Windows laptop and have chosen one of those operating systems to be Chromium OS. You did not make it clear if you built your own Chromium OS or are using CloudReady (as an example) or another option.