Turner, 28, joins from Farsley Celtic having previously been with Chester when they won the Conference North title.
Valentine spent the second half of last season on loan from Stalybridge Celtic, scoring against one of his former clubs, Scarborough Athletic.
Manager Paul Phillips said: “Valo did really well for us last season, he’s a good competitor, a threat going forward and he looks after himself fitness wise.
“He’s also a good voice in the changing room and with him knowing the club, his presence, willingness and workrate, I had no hesitation in asking him to sign for this season.”
Turner has recovered from an ACL operation but Phillips says that he “is now as fit as a butcher’s dog”.