Well I'm saying the each circular line has a power, and if the Sharingan had 3 circular lines the Rinnegan would be redundant. So the Sharingan got one line to hold the 3 tomoe's, and the Rinnegan got no tomoe's but all 3 lines. Naturally you put them together/fuse them you got something greater than what each can do, i.e 3 lines plus 9 tomoes.