The duo are also likely to feature heavily for Wales at next month's World Rugby Under-20 Championship, which is to be held in Manchester.
Though they have yet to appear in the Guinness PRO12, Evans and Hughes have racked up appearances in senior rugby with Carmarthen Quins and Llandovery respectively.
"We are delighted to confirm that both Dafydd and Shaun have signed their first professional contracts with us here at the Scarlets," said general manager of rugby Jon Daniels
"They both enjoyed a very successful Six Nations campaign and will take confidence into the forthcoming World Championships.
"We have a great relationship with our feeder club partners that allows young players like Shaun and Dafydd to continue their rugby development.
"We would like to wish them both the best of luck as they head to the Rugby World Championships."
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