Spence, 19, and Porritt, 24, both arrived at the Flamingo Land Stadium last season from Middlesbrough and Ossett United respectively.
Manager Darren Kelly said: "I'm delighted to have them both back at the club.
"I have known Luke for a while because he played for me at Hyde. He is a good player who offers pace and quality.
"Kian was brilliant for me when I took over as manager, showing versatility and impressing in every position he played.
"He will provide good competition in the middle of midfield, as well as other positions across the park."
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