Here in the earliest text, the infinitival marker to is twice repeated (“and to multiply and to till the earth”). The 1840 edition accidentally omitted the to before the verb multiply (but left the to before the verb till ). Subsequent editions have correctly ignored this 1840 omission. For additional examples where a repeated to has been omitted, see under 3 Nephi 4:23. The critical text will, of course, maintain the to before multiply here in Ether 6:18.
Summary: Maintain the multiple repetition of the infinitival to in Ether 6:18 (“they began to spread upon the face of the land and to multiply and to till the earth”), the reading of the earliest text.