Could this be used to hold acrylic sheets (8mm) for secondary glazing? Will I need both an A and B tape to magnetise to each other? Which tape would be best suited for this type of job?

You will need 25mm tape and yes it will need to go all the way around the sides of the acrylic sheet, with the window sill bearing the actual weight of the heavy sheet. You should use the Magnafix 25mm Tape Part A and Magnafix 25mm Tape Part B, A on the window frame and B on the acrylic so the two surfaces will be joined magnetically. Make sure the frame and the acrylic are clean before attaching the tapes, you shouldn’t have any problems removing the sheeting from the window frame.