Bellamy joins Psychologies from her current position as ad director of Hello! magazine, where she has worked for eight years, while Spires joins Red after a year's stint as ad director of Monocle.
The women will report directly to Tony Long, director of sales.
Bellamy, who started her career in classified sales at IPC Media, will begin her new position on October 1.
She said: "[Psychologies] is a refreshing and unique offering which has struck a chord with consumers, clearly demonstrated by its ever increasing ABC figures. I am very much looking forward to this exciting role and the new challenges it will present."
Both Bellamy and Spires worked for Hachette Filipacchi UK-owned titles Elle and Red in their earlier magazine careers.
Spires is due to rejoin Hachette on August 26.
She said: "Red is a fantastic brand that I have watched grow from strength to strength and I am very excited to be joining its great team once again."
Spires previously spent five years working at The National Magazine Company. During four of those years, she held the postion of ad manager of Harper's Bazaar and Harpers & Queen.
Long said: "We have worked hard over the last couple of years to create a distinct culture and team spirit at Hachette that has enabled us to build teams of the highest quality.
"We've been patient and ensured that we've got the right people in the right roles and I am absolutely delighted that Ruth and Tamsyn will be leading two of our fastest growing titles."