Combine apple one and icloud space - How to get 400Gb or 4Tb (for example)?

Dear All,

Despite a lot of reading, i'm still very unclear on how to combine icloud space made available via Apple One (200Gb in my case) and extra space from icloud.

I use 1 single iCloud AppleID.

I was using 200Gb of icloud+ and then subscribed to Apple One with 200Gb.

I then have 200Gb in total, and wish to pay the extra 2.99€ to get up to 400Gb.

However, from any of my iDevices, the only option I get tells me that I already have 200Gb via Apple One, and that I can upgrade my space to 2Tb. I see no "icloud" option to purchase 200Gb extra.

Can you please share guidance on how exactly keep my Apple One with 200Gb included, and pay to get 200Gb extra of icloud+, via the same AppleID?

Many thanks!