The invention relates to a double balance spring (21) including, made in a layer of silicon-based material (11), a first balance spring (23) coaxially mounted on a collet (13, 27), the collet (13, 27) including an extending part (9) that projects from said balance spring and which is made in a second layer of silicon-based material (5). According to the invention, said extending part extends (17) into a third layer (7) of silicon-based material coaxially with a second balance spring (25) so as to form a one-piece double balance spring (21) made of silicon-based materials. The invention also relates to a timepiece including a balance spring of this type and the method of manufacturing the same. The invention concerns the field of timepiece movements.