Ali Budynkiewicz, 58, and Lisa North, 48, known as the "Gangsta Grannies", entered the water at Samphire Hoe in Dover, Kent, at lunchtime on Wednesday.
They completed the swim in a time of 31 hours and 28 minutes on two-hour rotation shifts
They said: "We swam through the night and landed in France - turned around and swam straight back."
They added: "The swim was very tough. It really is a very long way, but we swam strong and steady. We feel so chuffed."
The swimmers, from Dorset, said their support team did a great job of keeping them fuelled with rice pudding, pork pies and bananas.
They completed the swim to raise money for a women's refuge supported by Bournemouth-based charity BCHA.
During the swim the pair were stung by jellyfish and encountered pods of dolphins and seals.
The Channel Swimming and Piloting Federation (CS&PF), which authorises and supports attempts, is due to formally ratify the timings.
Kevin Murphy of the CS&PF said it was believed to be the first time a team of two had completed the double crossing.
Two grandmothers successfully completed a 42-mile English Channel swim to France and back.