DMF solvent.  Chloroform solvent . They dissolve into each other.

Undecane is poorly soluble in water. In an aqueous solution of gadolinium acetate, it dissolves well. Gadolinium triacetate co-solvent. See our article.

Both are miscible to each other, I think you can use mixture of two as co-solvent.

There is a game of words and there are scientific generally accepted terms.

Yuri Mirgorod  and Mostafa Kamal Masud  thank you for advice. I will try