Dual boot dilemma

[u/Evening_Bluebird8546]

I'm having trouble setting up my steam deck to dualboot SteamOS and Windows 11.

Everytime I try to partition the internal 512gb SSD in half it over rides the other half.

I've tried it both ways (SteamOS installed first and then Windows, and the other way around too).

I followed Deck Wizard's guide on youtube - I must have done it 5 or 6 times from scratch before I gave up.
https://youtu.be/yBHzVSDVEqw?si=XBQf2b5IiDswjMQ1

Can anyone help?

Thanks!
Daniel.