Finnish word kaksois- comes from Proto-Uralic *kakta (Two.)

Detailed word origin of kaksois-

Dictionary entry	Language	Definition
*kakta	Proto-Uralic (urj-pro)	Two.
*kakci	Proto-Finnic (fiu-fin-pro)	Two.
kaksi	Finnish (fin)	Two.
kaksois-	Finnish (fin)	Double, binary, twin.

Words with the same origin as kaksois-