I want to create a map which should show two attributes of one layer (polygon shape), one as a hatching and the other as a normal color. I have already tried "Multiple Attributes" in the "Symbologie"-Tab in the layer options, but there I can only see the options "color ramp" and "symbol size".
The data types of the attributes are both text strings each with three to four value variations. That means a manageable amount of six combinations. I think I'll try your proposal of copying the layer. It seems a bit circumstantially and inelegantly though, so I'd be thankful for further suggestions.
If you only have 6 combinations, I suggest making a custom style with a polygon fill colour and a different form of hatching over your colours. Create a concatenated field or a 'class / category' field that you can name and apply your custom style to match.
Can you give an example of your data combinations.
For the case I described above I decided to create a custom style with multiple symbol layers. The Data Combinations were "yes"-, "no"- and "partly"-strings.
For another case which was more complex with regard to the number of combinations I adopted the first advise. I copied the layer and created two symbologies, one hatching and one color filling. As proposed I converted the legend to a graphic, worked fine.