Lock Craig, 22, joins the Saints from Leeds Carnegie and Northampton fans may remember him scoring against them in the LV= Cup back in February.
Spanish utility back Sempere is another newcomer who has played and scored at Franklin's Gardens recently - in a pre-season friendly for Nottingham last summer.
Craig said: "There are obviously a lot of people at Leeds who have helped me with my career and I owe them a lot of thanks, but now I'm massively looking forward to coming down to the Saints.
"The guys who I know here like Calum Clark and Scott Armstrong have told me that it's a great place to be, and visiting the club you can feel a buzz around the place, especially with the massive two or three weeks that are coming up. Hopefully I can help get the club to a similar place next year."
Sempere, 27, has been playing for Nottingham in the Championship this season but has experience in France's Top 14 competition with Montpellier.
"For me, the Saints is one of the best teams in the Premiership," he said.
"It is a great opportunity for me to show what I can do. I like to play with the ball in hand from everywhere, which is why I'm looking forward to playing in a team like Northampton. I enjoyed the game last August and the stadium seemed like a good place to play.
"Nottingham has been a good place to play and the club has good coaches and people there. It was my first experience in England and they made me feel very welcome. But the opportunity from Northampton was too good to turn down and I'm looking forward to playing there next season."
Director of rugby Jim Mallinder believes both players will be useful additions to the squad.
He said: "Our campaigns in the Premiership and in Europe have shown us the value of having a strong all-round squad, and with the World Cup coinciding with the start of next season we need players who can help us compete throughout next season.
"James and Craig have shown their quality in the Championship and in the games they have played against us in the last year, and we were impressed by what they can bring, not just in terms of playing ability but desire to succeed in a Saints shirt."
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