Hi Rene,
Based on your description, you want to link the 2 sheets with Specific Paste feature in SharePoint Online, when you change the content of one worksheet, the another sheet will change correspondingly.
However, due to some limitations of web browser technology, this is feature is not supported in Excel Online, you can refer to this article to know more information about copy and paste in Excel for Web: Excel for the web
However, as a workaround, we suggest you open these workbooks in Excel application-> Open -> From SharePoint site-> Open 2 worksheet-> Copy the content you want->Select another workbook-> Paste-> Special paste -> Paste Link. Then it will works.
Appreciate for your understanding. Please feel free to contact us if you need further help.
Regards,
Ivy